The Definitive Guide

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HTTP
The Definitive Guide
HTTP
The Definitive Guide
David Gourley and Brian Totty
with Marjorie Sayer, Sailu Reddy, and Anshu Aggarwal
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HTTP: The Definitive Guide
by David Gourley and Brian Totty
with Marjorie Sayer, Sailu Reddy, and Anshu Aggarwal
Copyright © 2002 O’Reilly Media, Inc. All rights reserved.
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[C] [01/08]
v
Table of Contents
Preface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xiii
Part I. HTTP: The Web’s Foundation
1. Overview of HTTP
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
HTTP: The Internet’s Multimedia Courier 3
Web Clients and Servers 4
Resources 4
Transactions 8
Messages 10
Connections 11
Protocol Versions 16
Architectural Components of the Web 17
The End of the Beginning 21
For More Information 21
2. URLs and Resources
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
Navigating the Internet’s Resources 24
URL Syntax 26
URL Shortcuts 30
Shady Characters 35
A Sea of Schemes 38
The Future 40
For More Information 41
3. HTTP Messages
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
The Flow of Messages 43
The Parts of a Message 44
vi | Table of Contents
Methods 53
Status Codes 59
Headers 67
For More Information 73
4. Connection Management
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
TCP Connections 74
TCP Performance Considerations 80
HTTP Connection Handling 86
Parallel Connections 88
Persistent Connections 90
Pipelined Connections 99
The Mysteries of Connection Close 101
For More Information 104
Part II. HTTP Architecture
5. Web Servers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
109
Web Servers Come in All Shapes and Sizes 109
A Minimal Perl Web Server 111
What Real Web Servers Do 113
Step 1: Accepting Client Connections 115
Step 2: Receiving Request Messages 116
Step 3: Processing Requests 120
Step 4: Mapping and Accessing Resources 120
Step 5: Building Responses 125
Step 6: Sending Responses 127
Step 7: Logging 127
For More Information 127
6. Proxies
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
129
Web Intermediaries 129
Why Use Proxies? 131
Where Do Proxies Go? 137
Client Proxy Settings 141
Tricky Things About Proxy Requests 144
Tracing Messages 150
Proxy Authentication 156
Table of Contents | vii
Proxy Interoperation 157
For More Information 160
7. Caching
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
161
Redundant Data Transfers 161
Bandwidth Bottlenecks 161
Flash Crowds 163
Distance Delays 163
Hits and Misses 164
Cache Topologies 168
Cache Processing Steps 171
Keeping Copies Fresh 175
Controlling Cachability 182
Setting Cache Controls 186
Detailed Algorithms 187
Caches and Advertising 194
For More Information 196
8. Integration Points: Gateways, Tunnels, and Relays
. . . . . . . . . . . . . . . . . . . .
197
Gateways 197
Protocol Gateways 200
Resource Gateways 203
Application Interfaces and Web Services 205
Tunnels 206
Relays 212
For More Information 213
9. Web Robots
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
215
Crawlers and Crawling 215
Robotic HTTP 225
Misbehaving Robots 228
Excluding Robots 229
Robot Etiquette 239
Search Engines 242
For More Information 246
10. HTTP-NG
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
247
HTTP’s Growing Pains 247
HTTP-NG Activity 248
viii | Table of Contents
Modularize and Enhance 248
Distributed Objects 249
Layer 1: Messaging 250
Layer 2: Remote Invocation 250
Layer 3: Web Application 251
WebMUX 251
Binary Wire Protocol 252
Current Status 252
For More Information 253
Part III. Identification, Authorization, and Security
11. Client Identification and Cookies
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
257
The Personal Touch 257
HTTP Headers 258
Client IP Address 259
User Login 260
Fat URLs 262
Cookies 263
For More Information 276
12. Basic Authentication
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
277
Authentication 277
Basic Authentication 281
The Security Flaws of Basic Authentication 283
For More Information 285
13. Digest Authentication
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
286
The Improvements of Digest Authentication 286
Digest Calculations 291
Quality of Protection Enhancements 299
Practical Considerations 300
Security Considerations 303
For More Information 306
14. Secure HTTP
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
307
Making HTTP Safe 307
Digital Cryptography 309
Table of Contents | ix
Symmetric-Key Cryptography 313
Public-Key Cryptography 315
Digital Signatures 317
Digital Certificates 319
HTTPS: The Details 322
A Real HTTPS Client 328
Tunneling Secure Traffic Through Proxies 335
For More Information 336
Part IV. Entities, Encodings, and Internationalization
15. Entities and Encodings
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
341
Messages Are Crates, Entities Are Cargo 342
Content-Length: The Entity’s Size 344
Entity Digests 347
Media Type and Charset 348
Content Encoding 351
Transfer Encoding and Chunked Encoding 354
Time-Varying Instances 359
Validators and Freshness 360
Range Requests 363
Delta Encoding 365
For More Information 369
16. Internationalization
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
370
HTTP Support for International Content 370
Character Sets and HTTP 371
Multilingual Character Encoding Primer 376
Language Tags and HTTP 384
Internationalized URIs 389
Other Considerations 392
For More Information 392
17. Content Negotiation and Transcoding
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
395
Content-Negotiation Techniques 395
Client-Driven Negotiation 396
Server-Driven Negotiation 397
Transparent Negotiation 400
x | Table of Contents
Transcoding 403
Next Steps 405
For More Information 406
Part V. Content Publishing and Distribution
18. Web Hosting
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
411
Hosting Services 411
Virtual Hosting 413
Making Web Sites Reliable 419
Making Web Sites Fast 422
For More Information 423
19. Publishing Systems
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
424
FrontPage Server Extensions for Publishing Support 424
WebDAV and Collaborative Authoring 429
For More Information 446
20. Redirection and Load Balancing
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
448
Why Redirect? 449
Where to Redirect 449
Overview of Redirection Protocols 450
General Redirection Methods 452
Proxy Redirection Methods 462
Cache Redirection Methods 469
Internet Cache Protocol 473
Cache Array Routing Protocol 475
Hyper Text Caching Protocol 478
For More Information 481
21. Logging and Usage Tracking
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
483
What to Log? 483
Log Formats 484
Hit Metering 492
A Word on Privacy 495
For More Information 495
Table of Contents | xi
Part VI. Appendixes
A. URI Schemes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
499
B. HTTP Status Codes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
505
C. HTTP Header Reference
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
508
D. MIME Types
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
533
E. Base-64 Encoding
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
570
F. Digest Authentication
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
574
G. Language Tags
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
581
H. MIME Charset Registry
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
602
Index
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
617
xiii
Preface
The Hypertext Transfer Protocol (HTTP) is the protocol programs use to communi-
cate over the World Wide Web. There are many applications of HTTP, but HTTP is
most famous for two-way conversation between web browsers and web servers.
HTTP began as a simple protocol, so you might think there really isn’t that much to
say about it. And yet here you stand, with a two-pound book in your hands. If you’re
wondering how we could have written 650 pages on HTTP, take a look at the Table
of Contents. This book isn’t just an HTTP header reference manual; it’s a veritable
bible of web architecture.
In this book, we try to tease apart HTTP’s interrelated and often misunderstood
rules, and we offer you a series of topic-based chapters that explain all the aspects of
HTTP. Throughout the book, we are careful to explain the “why” of HTTP, not just
the “how.” And to save you time chasing references, we explain many of the critical
non-HTTP technologies that are required to make HTTP applications work. You can
find the alphabetical header reference (which forms the basis of most conventional
HTTP texts) in a conveniently organized appendix. We hope this conceptual design
makes it easy for you to work with HTTP.
This book is written for anyone who wants to understand HTTP and the underlying
architecture of the Web. Software and hardware engineers can use this book as a
coherent reference for HTTP and related web technologies. Systems architects and
network administrators can use this book to better understand how to design,
deploy, and manage complicated web architectures. Performance engineers and ana-
lysts can benefit from the sections on caching and performance optimization. Mar-
keting and consulting professionals will be able to use the conceptual orientation to
better understand the landscape of web technologies.
This book illustrates common misconceptions, advises on “tricks of the trade,” pro-
vides convenient reference material, and serves as a readable introduction to dry and
confusing standards specifications. In a single book, we detail the essential and inter-
related technologies that make the Web work.
xiv |Preface
This book is the result of a tremendous amount of work by many people who share
an enthusiasm for Internet technologies. We hope you find it useful.
Running Example: Joe’s Hardware Store
Many of our chapters include a running example of a hypothetical online hardware
and home-improvement store called “Joe’s Hardware” to demonstrate technology
concepts. We have set up a real web site for the store (http://www.joes-hardware.
com) for you to test some of the examples in the book. We will maintain this web site
while this book remains in print.
Chapter-by-Chapter Guide
This book contains 21 chapters, divided into 5 logical parts (each with a technology
theme), and 8 useful appendixes containing reference data and surveys of related
technologies:
Part I, HTTP: The Web’s Foundation
Part II, HTTP Architecture
Part III, Identification, Authorization, and Security
Part IV, Entities, Encodings, and Internationalization
Part V, Content Publishing and Distribution
Part VI, Appendixes
Part I, HTTP: The Web’s Foundation,describes the core technology of HTTP, the
foundation of the Web, in four chapters:
• Chapter 1, Overview of HTTP, is a rapid-paced overview of HTTP.
• Chapter 2, URLs and Resources, details the formats of uniform resource locators
(URLs) and the various types of resources that URLs name across the Internet. It
also outlines the evolution to uniform resource names (URNs).
• Chapter 3, HTTP Messages, details how HTTP messages transport web content.
• Chapter 4, Connection Management, explains the commonly misunderstood and
poorly documented rules and behavior for managing HTTP connections.
Part II, HTTP Architecture, highlights the HTTP server, proxy, cache, gateway, and
robot applications that are the architectural building blocks of web systems. (Web
browsers are another building block, of course, but browsers already were covered
thoroughly in Part I of the book.) Part II contains the following six chapters:
• Chapter 5, Web Servers, gives an overview of web server architectures.
• Chapter 6, Proxies, explores HTTP proxy servers, which are intermediary serv-
ers that act as platforms for HTTP services and controls.
• Chapter 7, Caching, delves into the science of web caches—devices that improve
performance and reduce traffic by making local copies of popular documents.
Preface |xv
• Chapter 8, Integration Points: Gateways, Tunnels, and Relays, explains gateways
and application servers that allow HTTP to work with software that speaks dif-
ferent protocols, including Secure Sockets Layer (SSL) encrypted protocols.
• Chapter 9, Web Robots, describes the various types of clients that pervade the
Web, including the ubiquitous browsers, robots and spiders, and search engines.
• Chapter 10, HTTP-NG, talks about HTTP developments still in the works: the
HTTP-NG protocol.
Part III, Identification, Authorization, and Security, presents a suite of techniques and
technologies to track identity, enforce security, and control access to content. It con-
tains the following four chapters:
• Chapter 11, Client Identification and Cookies, talks about techniques to identify
users so that content can be personalized to the user audience.
• Chapter 12, Basic Authentication, highlights the basic mechanisms to verify user
identity. The chapter also examines how HTTP authentication interfaces with
databases.
• Chapter 13, Digest Authentication, explains digest authentication, a complex
proposed enhancement to HTTP that provides significantly enhanced security.
• Chapter 14, Secure HTTP, is a detailed overview of Internet cryptography, digi-
tal certificates, and SSL.
Part IV, Entities, Encodings, and Internationalization, focuses on the bodies of HTTP
messages (which contain the actual web content) and on the web standards that
describe and manipulate content stored in the message bodies. Part IV contains three
chapters:
• Chapter 15, Entities and Encodings, describes the structure of HTTP content.
• Chapter 16, Internationalization, surveys the web standards that allow users
around the globe to exchange content in different languages and character sets.
• Chapter 17, Content Negotiation and Transcoding, explains mechanisms for
negotiating acceptable content.
Part V, Content Publishing and Distribution, discusses the technology for publishing
and disseminating web content. It contains four chapters:
• Chapter 18, Web Hosting, discusses the ways people deploy servers in modern
web hosting environments and HTTP support for virtual web hosting.
• Chapter 19, Publishing Systems, discusses the technologies for creating web con-
tent and installing it onto web servers.
• Chapter 20, Redirection and Load Balancing, surveys the tools and techniques for
distributing incoming web traffic among a collection of servers.
• Chapter 21, Logging and Usage Tracking, covers log formats and common
questions.
xvi |Preface
Part VI, Appendixes, contains helpful reference appendixes and tutorials in related
technologies:
• Appendix A, URI Schemes, summarizes the protocols supported through uni-
form resource identifier (URI) schemes.
• Appendix B, HTTP Status Codes, conveniently lists the HTTP response codes.
• Appendix C, HTTP Header Reference, provides a reference list of HTTP header
fields.
• Appendix D, MIME Types, provides an extensive list of MIME types and
explains how MIME types are registered.
• Appendix E, Base-64 Encoding, explains base-64 encoding, used by HTTP
authentication.
• Appendix F, Digest Authentication, gives details on how to implement various
authentication schemes in HTTP.
• Appendix G, Language Tags, defines language tag values for HTTP language
headers.
• Appendix H, MIME Charset Registry, provides a detailed list of character encod-
ings, used for HTTP internationalization support.
Each chapter contains many examples and pointers to additional reference material.
Typographic Conventions
In this book, we use the following typographic conventions:
Italic
Used for URLs, C functions, command names, MIME types, new terms where
they are defined, and emphasis
Constant width
Used for computer output, code, and any literal text
Constant width bold
Used for user input
Comments and Questions
Please address comments and questions concerning this book to the publisher:
O’Reilly & Associates, Inc.
1005 Gravenstein Highway North
Sebastopol, CA 95472
(800) 998-9938 (in the United States or Canada)
(707) 829-0515 (international/local)
(707) 829-0104 (fax)
Preface |xvii
There is a web page for this book, which lists errata, examples, or any additional
information. You can access this page at:
http://www.oreilly.com/catalog/httptdg/
To comment or ask technical questions about this book, send email to:
bookquestions@oreilly.com
For more information about books, conferences, Resource Centers, and the O’Reilly
Network, see the O’Reilly web site at:
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Acknowledgments
This book is the labor of many. The five authors would like to hold up a few people
in thanks for their significant contributions to this project.
To start, we’d like to thank Linda Mui, our editor at O’Reilly. Linda first met with
David and Brian way back in 1996, and she refined and steered several concepts into
the book you hold today. Linda also helped keep our wandering gang of first-time
book authors moving in a coherent direction and on a progressing (if not rapid) time-
line. Most of all, Linda gave us the chance to create this book. We’re very grateful.
We’d also like to thank several tremendously bright, knowledgeable, and kind souls
who devoted noteworthy energy to reviewing, commenting on, and correcting drafts
of this book. These include Tony Bourke, Sean Burke, Mike Chowla, Shernaz Daver,
Fred Douglis, Paula Ferguson, Vikas Jha, Yves Lafon, Peter Mattis, Chuck Neer-
daels, Luis Tavera, Duane Wessels, Dave Wu, and Marco Zagha. Their viewpoints
and suggestions have improved the book tremendously.
Rob Romano from O’Reilly created most of the amazing artwork you’ll find in this
book. The book contains an unusually large number of detailed illustrations that
make subtle concepts very clear. Many of these illustrations were painstakingly cre-
ated and revised numerous times. If a picture is worth a thousand words, Rob added
hundreds of pages of value to this book.
Brian would like to personally thank all of the authors for their dedication to this
project. A tremendous amount of time was invested by the authors in a challenge to
make the first detailed but accessible treatment of HTTP. Weddings, childbirths,
killer work projects, startup companies, and graduate schools intervened, but the
authors held together to bring this project to a successful completion. We believe the
result is worthy of everyone’s hard work and, most importantly, that it provides a
valuable service. Brian also would like to thank the employees of Inktomi for their
enthusiasm and support and for their deep insights about the use of HTTP in real-
world applications. Also, thanks to the fine folks at Cajun-shop.com for allowing us
to use their site for some of the examples in this book.
xviii |Preface
David would like to thank his family, particularly his mother and grandfather for
their ongoing support. He’d like to thank those that have put up with his erratic
schedule over the years writing the book. He’d also like to thank Slurp, Orctomi, and
Norma for everything they’ve done, and his fellow authors for all their hard work.
Finally, he would like to thank Brian for roping him into yet another adventure.
Marjorie would like to thank her husband, Alan Liu, for technical insight, familial
support and understanding. Marjorie thanks her fellow authors for many insights
and inspirations. She is grateful for the experience of working together on this book.
Sailu would like to thank David and Brian for the opportunity to work on this book,
and Chuck Neerdaels for introducing him to HTTP.
Anshu would like to thank his wife, Rashi, and his parents for their patience, sup-
port, and encouragement during the long years spent writing this book.
Finally, the authors collectively thank the famous and nameless Internet pioneers,
whose research, development, and evangelism over the past four decades contrib-
uted so much to our scientific, social, and economic community. Without these
labors, there would be no subject for this book.
PART I
HTTP: The Web’s Foundation
This section is an introduction to the HTTP protocol. The next four chapters
describe the core technology of HTTP, the foundation of the Web:
• Chapter 1, Overview of HTTP, is a rapid-paced overview of HTTP.
• Chapter 2, URLs and Resources, details the formats of URLs and the various
types of resources that URLs name across the Internet. We also outline the evo-
lution to URNs.
• Chapter 3, HTTP Messages, details the HTTP messages that transport web
content.
• Chapter 4, Connection Management, discusses the commonly misunderstood
and poorly documented rules and behavior for managing TCP connections by
HTTP.
3
CHAPTER 1
Overview of HTTP
The world’s web browsers, servers, and related web applications all talk to each
other through HTTP, the Hypertext Transfer Protocol. HTTP is the common lan-
guage of the modern global Internet.
This chapter is a concise overview of HTTP. You’ll see how web applications use
HTTP to communicate, and you’ll get a rough idea of how HTTP does its job. In
particular, we talk about:
• How web clients and servers communicate
• Where resources (web content) come from
• How web transactions work
• The format of the messages used for HTTP communication
• The underlying TCP network transport
• The different variations of the HTTP protocol
• Some of the many HTTP architectural components installed around the Internet
We’ve got a lot of ground to cover, so let’s get started on our tour of HTTP.
HTTP: The Internet’s Multimedia Courier
Billions of JPEG images, HTML pages, text files, MPEG movies, WAV audio files,
Java applets, and more cruise through the Internet each and every day. HTTP moves
the bulk of this information quickly, conveniently, and reliably from web servers all
around the world to web browsers on people’s desktops.
Because HTTP uses reliable data-transmission protocols, it guarantees that your data
will not be damaged or scrambled in transit, even when it comes from the other side of
the globe. This is good for you as a user, because you can access information without
worrying about its integrity. Reliable transmission is also good for you as an Internet
application developer, because you don’t have to worry about HTTP communications
4|Chapter 1: Overview of HTTP
being destroyed, duplicated, or distorted in transit. You can focus on programming
the distinguishing details of your application, without worrying about the flaws and
foibles of the Internet.
Let’s look more closely at how HTTP transports the Web’s traffic.
Web Clients and Servers
Web content lives on web servers. Web servers speak the HTTP protocol, so they are
often called HTTP servers. These HTTP servers store the Internet’s data and provide
the data when it is requested by HTTP clients. The clients send HTTP requests to
servers, and servers return the requested data in HTTP responses, as sketched in
Figure 1-1. Together, HTTP clients and HTTP servers make up the basic compo-
nents of the World Wide Web.
You probably use HTTP clients every day. The most common client is a web
browser, such as Microsoft Internet Explorer or Netscape Navigator. Web browsers
request HTTP objects from servers and display the objects on your screen.
When you browse to a page, such as “http://www.oreilly.com/index.html,” your
browser sends an HTTP request to the server www.oreilly.com (see Figure 1-1). The
server tries to find the desired object (in this case, “/index.html”) and, if successful,
sends the object to the client in an HTTP response, along with the type of the object,
the length of the object, and other information.
Resources
Web servers host web resources. A web resource is the source of web content. The
simplest kind of web resource is a static file on the web server’s filesystem. These
files can contain anything: they might be text files, HTML files, Microsoft Word
files, Adobe Acrobat files, JPEG image files, AVI movie files, or any other format you
can think of.
However, resources don’t have to be static files. Resources can also be software pro-
grams that generate content on demand. These dynamic content resources can gen-
erate content based on your identity, on what information you’ve requested, or on
Figure 1-1. Web clients and servers
HTTP request
“Get me the document called /index.html.”
Client Server
www.oreilly.com
HTTP response
“Okay, here it is, it’s in HTML format and is 3,150 characters long.”
Resources |5
the time of day. They can show you a live image from a camera, or let you trade
stocks, search real estate databases, or buy gifts from online stores (see Figure 1-2).
In summary, a resource is any kind of content source. A file containing your com-
pany’s sales forecast spreadsheet is a resource. A web gateway to scan your local
public library’s shelves is a resource. An Internet search engine is a resource.
Media Types
Because the Internet hosts many thousands of different data types, HTTP carefully
tags each object being transported through the Web with a data format label called a
MIME type. MIME (Multipurpose Internet Mail Extensions) was originally designed
to solve problems encountered in moving messages between different electronic mail
systems. MIME worked so well for email that HTTP adopted it to describe and label
its own multimedia content.
Web servers attach a MIME type to all HTTP object data (see Figure 1-3). When a
web browser gets an object back from a server, it looks at the associated MIME type
to see if it knows how to handle the object. Most browsers can handle hundreds of
popular object types: displaying image files, parsing and formatting HTML files,
playing audio files through the computer’s speakers, or launching external plug-in
software to handle special formats.
Figure 1-2. A web resource is anything that provides web content
Client Server
Internet
E-commerce
gateway
Real estate search
gateway
Stock trading
gateway
Web cam
gateway
11000101101
Image file
Text file
Filesystem Resources
6|Chapter 1: Overview of HTTP
A MIME type is a textual label, represented as a primary object type and a specific
subtype, separated by a slash. For example:
• An HTML-formatted text document would be labeled with type text/html.
• A plain ASCII text document would be labeled with type text/plain.
• A JPEG version of an image would be image/jpeg.
• A GIF-format image would be image/gif.
• An Apple QuickTime movie would be video/quicktime.
• A Microsoft PowerPoint presentation would be application/vnd.ms-powerpoint.
There are hundreds of popular MIME types, and many more experimental or limited-
use types. A very thorough MIME type list is provided in Appendix D.
URIs
Each web server resource has a name, so clients can point out what resources they
are interested in. The server resource name is called a uniform resource identifier,or
URI. URIs are like the postal addresses of the Internet, uniquely identifying and
locating information resources around the world.
Here’s a URI for an image resource on Joe’s Hardware store’s web server:
http://www.joes-hardware.com/specials/saw-blade.gif
Figure 1-4 shows how the URI specifies the HTTP protocol to access the saw-blade
GIF resource on Joe’s store’s server. Given the URI, HTTP can retrieve the object.
URIs come in two flavors, called URLs and URNs. Let’s take a peek at each of these
types of resource identifiers now.
URLs
The uniform resource locator (URL) is the most common form of resource identifier.
URLs describe the specific location of a resource on a particular server. They tell you
exactly how to fetch a resource from a precise, fixed location. Figure 1-4 shows how
a URL tells precisely where a resource is located and how to access it. Table 1-1
shows a few examples of URLs.
Figure 1-3. MIME types are sent back with the data content
Client Server
Content-type: image/jpeg
Content-length: 12984
MIME type
Resources |7
Most URLs follow a standardized format of three main parts:
• The first part of the URL is called the scheme, and it describes the protocol used
to access the resource. This is usually the HTTP protocol (http://).
• The second part gives the server Internet address (e.g., www.joes-hardware.com).
• The rest names a resource on the web server (e.g., /specials/saw-blade.gif ).
Today, almost every URI is a URL.
URNs
The second flavor of URI is the uniform resource name, or URN. A URN serves as a
unique name for a particular piece of content, independent of where the resource
currently resides. These location-independent URNs allow resources to move from
place to place. URNs also allow resources to be accessed by multiple network access
protocols while maintaining the same name.
For example, the following URN might be used to name the Internet standards docu-
ment “RFC 2141” regardless of where it resides (it may even be copied in several
places):
urn:ietf:rfc:2141
Figure 1-4. URLs specify protocol, server, and local resource
Table 1-1. Example URLs
URL Description
http://www.oreilly.com/index.html The home URL for O’Reilly & Associates, Inc.
http://www.yahoo.com/images/logo.gif The URL for the Yahoo! web site’s logo
http://www.joes-hardware.com/inventory-check.
cgi?item=12731
The URL for a program that checks if inventory item
#12731 is in stock
ftp://joe:tools4u@ftp.joes-hardware.com/locking-
pliers.gif
The URL for the locking-pliers.gif image file, using
password-protected FTP as the access protocol
Client www.joes-hardware.com
Content-type: image/gif
Content-length: 8572
http://www.joes-hardware.com/specials/saw-blade.gif
Use HTTP protocol Go to www.joes-hardware.com Grab the resource called /specials/saw-blade.gif
12 3
8|Chapter 1: Overview of HTTP
URNs are still experimental and not yet widely adopted. To work effectively, URNs
need a supporting infrastructure to resolve resource locations; the lack of such an
infrastructure has also slowed their adoption. But URNs do hold some exciting
promise for the future. We’ll discuss URNs in a bit more detail in Chapter 2, but
most of the remainder of this book focuses almost exclusively on URLs.
Unless stated otherwise, we adopt the conventional terminology and use URI and
URL interchangeably for the remainder of this book.
Transactions
Let’s look in more detail how clients use HTTP to transact with web servers and
their resources. An HTTP transaction consists of a request command (sent from cli-
ent to server), and a response result (sent from the server back to the client). This
communication happens with formatted blocks of data called HTTP messages,as
illustrated in Figure 1-5.
Methods
HTTP supports several different request commands, called HTTP methods. Every
HTTP request message has a method. The method tells the server what action to per-
form (fetch a web page, run a gateway program, delete a file, etc.). Table 1-2 lists five
common HTTP methods.
Figure 1-5. HTTP transactions consist of request and response messages
Table 1-2. Some common HTTP methods
HTTP method Description
GET Send named resource from the server to the client.
PUT Store data from client into a named server resource.
Internet
HTTP request message contains
the command and the URI
GET /specials/saw-blade.gif HTTP/1.0
Host: www.joes-hardware.com
Client www.joes-hardware.com
HTTP/1.0 200 OK
Content-type: image/gif
Content-length: 8572 HTTP response message contains
the result of the transaction
Transactions |9
We’ll discuss HTTP methods in detail in Chapter 3.
Status Codes
Every HTTP response message comes back with a status code. The status code is a
three-digit numeric code that tells the client if the request succeeded, or if other
actions are required. A few common status codes are shown in Table 1-3.
HTTP also sends an explanatory textual “reason phrase” with each numeric status
code (see the response message in Figure 1-5). The textual phrase is included only for
descriptive purposes; the numeric code is used for all processing.
The following status codes and reason phrases are treated identically by HTTP soft-
ware:
200 OK
200 Document attached
200 Success
200 All’s cool, dude
HTTP status codes are explained in detail in Chapter 3.
Web Pages Can Consist of Multiple Objects
An application often issues multiple HTTP transactions to accomplish a task. For
example, a web browser issues a cascade of HTTP transactions to fetch and display a
graphics-rich web page. The browser performs one transaction to fetch the HTML
“skeleton” that describes the page layout, then issues additional HTTP transactions
for each embedded image, graphics pane, Java applet, etc. These embedded
resources might even reside on different servers, as shown in Figure 1-6. Thus, a
“web page” often is a collection of resources, not a single resource.
DELETE Delete the named resource from a server.
POST Send client data into a server gateway application.
HEAD Send just the HTTP headers from the response for the named resource.
Table 1-3. Some common HTTP status codes
HTTP status code Description
200 OK. Document returned correctly.
302 Redirect. Go someplace else to get the resource.
404 Not Found. Can’t find this resource.
Table 1-2. Some common HTTP methods (continued)
HTTP method Description
10 |Chapter 1: Overview of HTTP
Messages
Now let’s take a quick look at the structure of HTTP request and response mes-
sages. We’ll study HTTP messages in exquisite detail in Chapter 3.
HTTP messages are simple, line-oriented sequences of characters. Because they are
plain text, not binary, they are easy for humans to read and write.*Figure 1-7 shows
the HTTP messages for a simple transaction.
HTTP messages sent from web clients to web servers are called request messages.
Messages from servers to clients are called response messages. There are no other
kinds of HTTP messages. The formats of HTTP request and response messages are
very similar.
Figure 1-6. Composite web pages require separate HTTP transactions for each embedded resource
* Some programmers complain about the difficulty of HTTP parsing, which can be tricky and error-prone,
especially when designing high-speed software. A binary format or a more restricted text format might have
been simpler to process, but most HTTP programmers appreciate HTTP’s extensibility and debuggability.
Figure 1-7. HTTP messages have a simple, line-oriented text structure
Client
Server 1
Server 2
Internet
GET /test/hi-there.txt HTTP/1.0
Accept: text/*
Accept-Language: en,fr
HTTP/1.0 200 OK
Content-type: text/plain
Content-length: 19
Hi! I’m a message!
Start line
Headers
Body
(a) Request message (b) Response message
Connections |11
HTTP messages consist of three parts:
Start line
The first line of the message is the start line, indicating what to do for a request
or what happened for a response.
Header fields
Zero or more header fields follow the start line. Each header field consists of a
name and a value, separated by a colon (:) for easy parsing. The headers end
with a blank line. Adding a header field is as easy as adding another line.
Body
After the blank line is an optional message body containing any kind of data.
Request bodies carry data to the web server; response bodies carry data back to
the client. Unlike the start lines and headers, which are textual and structured,
the body can contain arbitrary binary data (e.g., images, videos, audio tracks,
software applications). Of course, the body can also contain text.
Simple Message Example
Figure 1-8 shows the HTTP messages that might be sent as part of a simple transac-
tion. The browser requests the resource http://www.joes-hardware.com/tools.html.
In Figure 1-8, the browser sends an HTTP request message. The request has a GET
method in the start line, and the local resource is /tools.html. The request indicates it
is speaking Version 1.0 of the HTTP protocol. The request message has no body,
because no request data is needed to GET a simple document from a server.
The server sends back an HTTP response message. The response contains the HTTP
version number (HTTP/1.0), a success status code (200), a descriptive reason phrase
(OK), and a block of response header fields, all followed by the response body con-
taining the requested document. The response body length is noted in the Content-
Length header, and the document’s MIME type is noted in the Content-Type
header.
Connections
Now that we’ve sketched what HTTP’s messages look like, let’s talk for a moment
about how messages move from place to place, across Transmission Control Protocol
(TCP) connections.
TCP/IP
HTTP is an application layer protocol. HTTP doesn’t worry about the nitty-gritty
details of network communication; instead, it leaves the details of networking to
TCP/IP, the popular reliable Internet transport protocol.
12 |Chapter 1: Overview of HTTP
TCP provides:
• Error-free data transportation
• In-order delivery (data will always arrive in the order in which it was sent)
• Unsegmented data stream (can dribble out data in any size at any time)
The Internet itself is based on TCP/IP, a popular layered set of packet-switched net-
work protocols spoken by computers and network devices around the world. TCP/IP
hides the peculiarities and foibles of individual networks and hardware, letting com-
puters and networks of any type talk together reliably.
Once a TCP connection is established, messages exchanged between the client and
server computers will never be lost, damaged, or received out of order.
In networking terms, the HTTP protocol is layered over TCP. HTTP uses TCP to
transport its message data. Likewise, TCP is layered over IP (see Figure 1-9).
Figure 1-8. Example GET transaction for http://www.joes-hardware.com/tools.html
GET /tools.html HTTP/1.0
User-agent: Mozilla/4.75 [en] (Win98; U)
Host: www.joes-hardware.com
Accept: text/html, image/gif, image/jpeg
Accept-language: en
HTTP/1.0 200 OK
Date: Sun, o1 Oct 2000 23:25:17 GMT
Server: Apache/1.3.11 BSafe-SSL/1.38 (Unix)
Last-modified: Tue, 04 Jul 2000 09:46:21 GMT
Content-length: 403
Content-type: text/html
<HTML>
<HEAD><TITLE>Joe’s Tools</TITLE></HEAD>
<BODY>
<H1>Tools Page</H1>
<H2>Hammers</H2>
<P>Joe’s Hardware Online has the largest selection of
hammers on the earth.</P>
<H2><A NAME=drills></A>Drills</H2>
<P>Joe’s Hardware has a complete line of cordless
and corded drills, as well as the latest in
plutonium-powered atomic drills, for those big
around the house jobs./<P>...
</BODY>
</HTML>
Client www.joes-hardware.com
(a) Request message
(b) Response message
Request start line (command)
Request headers
No request body
Response start line
(status)
Response headers
Response body
Connections |13
Connections, IP Addresses, and Port Numbers
Before an HTTP client can send a message to a server, it needs to establish a TCP/IP
connection between the client and server using Internet protocol (IP) addresses and
port numbers.
Setting up a TCP connection is sort of like calling someone at a corporate office.
First, you dial the company’s phone number. This gets you to the right organization.
Then, you dial the specific extension of the person you’re trying to reach.
In TCP, you need the IP address of the server computer and the TCP port number
associated with the specific software program running on the server.
This is all well and good, but how do you get the IP address and port number of the
HTTP server in the first place? Why, the URL, of course! We mentioned before that
URLs are the addresses for resources, so naturally enough they can provide us with
the IP address for the machine that has the resource. Let’s take a look at a few URLs:
http://207.200.83.29:80/index.html
http://www.netscape.com:80/index.html
http://www.netscape.com/index.html
The first URL has the machine’s IP address, “207.200.83.29”, and port number,
“80”.
The second URL doesn’t have a numeric IP address; it has a textual domain name, or
hostname (“www.netscape.com”). The hostname is just a human-friendly alias for an
IP address. Hostnames can easily be converted into IP addresses through a facility
called the Domain Name Service (DNS), so we’re all set here, too. We will talk much
more about DNS and URLs in Chapter 2.
The final URL has no port number. When the port number is missing from an HTTP
URL, you can assume the default value of port 80.
With the IP address and port number, a client can easily communicate via TCP/IP.
Figure 1-10 shows how a browser uses HTTP to display a simple HTML resource
that resides on a distant server.
Figure 1-9. HTTP network protocol stack
HTTP Application layer
TCP Transport layer
IP Network layer
Network-specific link interface Data link layer
Physical network hardware Physical layer
14 |Chapter 1: Overview of HTTP
Here are the steps:
(a) The browser extracts the server’s hostname from the URL.
(b) The browser converts the server’s hostname into the server’s IP address.
(c) The browser extracts the port number (if any) from the URL.
(d) The browser establishes a TCP connection with the web server.
(e) The browser sends an HTTP request message to the server.
(f) The server sends an HTTP response back to the browser.
(g) The connection is closed, and the browser displays the document.
Figure 1-10. Basic browser connection process
Client Server
Internet
(d) Connect to 161.58.228.45 port 80
Client Server
Internet
(e) Send an HTTP GET request
Client Server
Internet
(f) Read HTTP response from server
Client Server
Internet
(g) Close the connection
User types in URL
http://www.joes-hardware.com:80/tools.html
(c) Get the port number (80)
www.joes-hardware.com
(a) Get the hostname
(b) DNS
Browser showing page
Connections |15
A Real Example Using Telnet
Because HTTP uses TCP/IP, and is text-based, as opposed to using some obscure
binary format, it is simple to talk directly to a web server.
The Telnet utility connects your keyboard to a destination TCP port and connects
the TCP port output back to your display screen. Telnet is commonly used for
remote terminal sessions, but it can generally connect to any TCP server, including
HTTP servers.
You can use the Telnet utility to talk directly to web servers. Telnet lets you open a
TCP connection to a port on a machine and type characters directly into the port.
The web server treats you as a web client, and any data sent back on the TCP con-
nection is displayed onscreen.
Let’s use Telnet to interact with a real web server. We will use Telnet to fetch the
document pointed to by the URL http://www.joes-hardware.com:80/tools.html (you
can try this example yourself).
Let’s review what should happen:
• First, we need to look up the IP address of www.joes-hardware.com and open a
TCP connection to port 80 on that machine. Telnet does this legwork for us.
• Once the TCP connection is open, we need to type in the HTTP request.
• When the request is complete (indicated by a blank line), the server should send
back the content in an HTTP response and close the connection.
Our example HTTP request for http://www.joes-hardware.com:80/tools.html is shown
in Example 1-1. What we typed is shown in boldface.
Example 1-1. An HTTP transaction using telnet
%telnet www.joes-hardware.com 80
Trying 161.58.228.45...
Connected to joes-hardware.com.
Escape character is '^]'.
GET /tools.html HTTP/1.1
Host: www.joes-hardware.com
HTTP/1.1 200 OK
Date: Sun, 01 Oct 2000 23:25:17 GMT
Server: Apache/1.3.11 BSafe-SSL/1.38 (Unix) FrontPage/4.0.4.3
Last-Modified: Tue, 04 Jul 2000 09:46:21 GMT
ETag: "373979-193-3961b26d"
Accept-Ranges: bytes
Content-Length: 403
Connection: close
Content-Type: text/html
16 |Chapter 1: Overview of HTTP
Telnet looks up the hostname and opens a connection to the www.joes-hardware.com
web server, which is listening on port 80. The three lines after the command are out-
put from Telnet, telling us it has established a connection.
We then type in our basic request command, “GET /tools.html HTTP/1.1”, and send
a Host header providing the original hostname, followed by a blank line, asking the
server to GET us the resource “/tools.html” from the server www.joes-hardware.com.
After that, the server responds with a response line, several response headers, a blank
line, and finally the body of the HTML document.
Beware that Telnet mimics HTTP clients well but doesn’t work well as a server.
And automated Telnet scripting is no fun at all. For a more flexible tool, you
might want to check out nc (netcat). The nc tool lets you easily manipulate and
script UDP- and TCP-based traffic, including HTTP. See http://netcat.
sourceforge.net for details.
Protocol Versions
There are several versions of the HTTP protocol in use today. HTTP applications
need to work hard to robustly handle different variations of the HTTP protocol. The
versions in use are:
HTTP/0.9
The 1991 prototype version of HTTP is known as HTTP/0.9. This protocol con-
tains many serious design flaws and should be used only to interoperate with
legacy clients. HTTP/0.9 supports only the GET method, and it does not sup-
port MIME typing of multimedia content, HTTP headers, or version numbers.
HTTP/0.9 was originally defined to fetch simple HTML objects. It was soon
replaced with HTTP/1.0.
HTTP/1.0
1.0 was the first version of HTTP that was widely deployed. HTTP/1.0 added
version numbers, HTTP headers, additional methods, and multimedia object
handling. HTTP/1.0 made it practical to support graphically appealing web
<HTML>
<HEAD><TITLE>Joe's Tools</TITLE></HEAD>
<BODY>
<H1>Tools Page</H1>
<H2>Hammers</H2>
<P>Joe's Hardware Online has the largest selection of hammers on the earth.</P>
<H2><A NAME=drills></A>Drills</H2>
<P>Joe's Hardware has a complete line of cordless and corded drills, as well as the latest
in plutonium-powered atomic drills, for those big around the house jobs.</P> ...
</BODY>
</HTML>
Connection closed by foreign host.
Example 1-1. An HTTP transaction using telnet (continued)
Architectural Components of the Web |17
pages and interactive forms, which helped promote the wide-scale adoption of
the World Wide Web. This specification was never well specified. It represented
a collection of best practices in a time of rapid commercial and academic evolu-
tion of the protocol.
HTTP/1.0+
Many popular web clients and servers rapidly added features to HTTP in the
mid-1990s to meet the demands of a rapidly expanding, commercially success-
ful World Wide Web. Many of these features, including long-lasting “keep-
alive” connections, virtual hosting support, and proxy connection support, were
added to HTTP and became unofficial, de facto standards. This informal,
extended version of HTTP is often referred to as HTTP/1.0+.
HTTP/1.1
HTTP/1.1 focused on correcting architectural flaws in the design of HTTP, spec-
ifying semantics, introducing significant performance optimizations, and remov-
ing mis-features. HTTP/1.1 also included support for the more sophisticated
web applications and deployments that were under way in the late 1990s.
HTTP/1.1 is the current version of HTTP.
HTTP-NG (a.k.a. HTTP/2.0)
HTTP-NG is a prototype proposal for an architectural successor to HTTP/1.1
that focuses on significant performance optimizations and a more powerful frame-
work for remote execution of server logic. The HTTP-NG research effort con-
cluded in 1998, and at the time of this writing, there are no plans to advance this
proposal as a replacement for HTTP/1.1. See Chapter 10 for more information.
Architectural Components of the Web
In this overview chapter, we’ve focused on how two web applications (web browsers
and web servers) send messages back and forth to implement basic transactions.
There are many other web applications that you interact with on the Internet. In this
section, we’ll outline several other important applications, including:
Proxies
HTTP intermediaries that sit between clients and servers
Caches
HTTP storehouses that keep copies of popular web pages close to clients
Gateways
Special web servers that connect to other applications
Tunnels
Special proxies that blindly forward HTTP communications
Agents
Semi-intelligent web clients that make automated HTTP requests
18 |Chapter 1: Overview of HTTP
Proxies
Let’s start by looking at HTTP proxy servers, important building blocks for web
security, application integration, and performance optimization.
As shown in Figure 1-11, a proxy sits between a client and a server, receiving all of
the client’s HTTP requests and relaying the requests to the server (perhaps after
modifying the requests). These applications act as a proxy for the user, accessing the
server on the user’s behalf.
Proxies are often used for security, acting as trusted intermediaries through which all
web traffic flows. Proxies can also filter requests and responses; for example, to
detect application viruses in corporate downloads or to filter adult content away
from elementary-school students. We’ll talk about proxies in detail in Chapter 6.
Caches
Aweb cache or caching proxy is a special type of HTTP proxy server that keeps cop-
ies of popular documents that pass through the proxy. The next client requesting the
same document can be served from the cache’s personal copy (see Figure 1-12).
Figure 1-11. Proxies relay traffic between client and server
Figure 1-12. Caching proxies keep local copies of popular documents to improve performance
Client Server
Internet
Proxy
Client Server
Internet
Proxy cache
Client
Architectural Components of the Web |19
A client may be able to download a document much more quickly from a nearby
cache than from a distant web server. HTTP defines many facilities to make caching
more effective and to regulate the freshness and privacy of cached content. We cover
caching technology in Chapter 7.
Gateways
Gateways are special servers that act as intermediaries for other servers. They are
often used to convert HTTP traffic to another protocol. A gateway always receives
requests as if it was the origin server for the resource. The client may not be aware it
is communicating with a gateway.
For example, an HTTP/FTP gateway receives requests for FTP URIs via HTTP
requests but fetches the documents using the FTP protocol (see Figure 1-13). The
resulting document is packed into an HTTP message and sent to the client. We dis-
cuss gateways in Chapter 8.
Tunnels
Tunnels are HTTP applications that, after setup, blindly relay raw data between two
connections. HTTP tunnels are often used to transport non-HTTP data over one or
more HTTP connections, without looking at the data.
One popular use of HTTP tunnels is to carry encrypted Secure Sockets Layer (SSL)
traffic through an HTTP connection, allowing SSL traffic through corporate fire-
walls that permit only web traffic. As sketched in Figure 1-14, an HTTP/SSL tunnel
receives an HTTP request to establish an outgoing connection to a destination
address and port, then proceeds to tunnel the encrypted SSL traffic over the HTTP
channel so that it can be blindly relayed to the destination server.
Agents
User agents (or just agents) are client programs that make HTTP requests on the
user’s behalf. Any application that issues web requests is an HTTP agent. So far,
we’ve talked about only one kind of HTTP agent: web browsers. But there are many
other kinds of user agents.
Figure 1-13. HTTP/FTP gateway
HTTP client FTP serverHTTP/FTP
gateway
HTTP FTP
20 |Chapter 1: Overview of HTTP
For example, there are machine-automated user agents that autonomously wander
the Web, issuing HTTP transactions and fetching content, without human supervi-
sion. These automated agents often have colorful names, such as “spiders” or “web
robots” (see Figure 1-15). Spiders wander the Web to build useful archives of web
content, such as a search engine’s database or a product catalog for a comparison-
shopping robot. See Chapter 9 for more information.
Figure 1-14. Tunnels forward data across non-HTTP networks (HTTP/SSL tunnel shown)
Figure 1-15. Automated search engine “spiders” are agents, fetching web pages around the world
Server
Client
SSL
Tunnel start
SSLHTTP HTTP
connection SSLHTTP
SSL
Tunnel endpoint
Port 80
SSL
connection SSL
Port 443
Search engine
“spider”
Web serverWeb serverWeb server
Search engine
database
For More Information |21
The End of the Beginning
That’s it for our quick introduction to HTTP. In this chapter, we highlighted HTTP’s
role as a multimedia transport protocol. We outlined how HTTP uses URIs to name
multimedia resources on remote servers, we sketched how HTTP request and
response messages are used to manipulate multimedia resources on remote servers,
and we finished by surveying a few of the web applications that use HTTP.
The remaining chapters explain the technical machinery of the HTTP protocol,
applications, and resources in much more detail.
For More Information
Later chapters of this book will explore HTTP in much more detail, but you might
find that some of the following sources contain useful background about particular
topics we covered in this chapter.
HTTP Protocol Information
HTTP Pocket Reference
Clinton Wong, O’Reilly & Associates, Inc. This little book provides a concise
introduction to HTTP and a quick reference to each of the headers and status
codes that compose HTTP transactions.
http://www.w3.org/Protocols/
This W3C web page contains many great links about the HTTP protocol.
http://www.ietf.org/rfc/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol—HTTP/1.1,” is the official specifica-
tion for HTTP/1.1, the current version of the HTTP protocol. The specification
is a well-written, well-organized, detailed reference for HTTP, but it isn’t ideal
for readers who want to learn the underlying concepts and motivations of HTTP
or the differences between theory and practice. We hope that this book fills in
the underlying concepts, so you can make better use of the specification.
http://www.ietf.org/rfc/rfc1945.txt
RFC 1945, “Hypertext Transfer Protocol—HTTP/1.0,” is an informational RFC
that describes the modern foundation for HTTP. It details the officially sanc-
tioned and “best-practice” behavior of web applications at the time the specifica-
tion was written. It also contains some useful descriptions about behavior that is
deprecated in HTTP/1.1 but still widely implemented by legacy applications.
http://www.w3.org/Protocols/HTTP/AsImplemented.html
This web page contains a description of the 1991 HTTP/0.9 protocol, which
implements only GET requests and has no content typing.
22 |Chapter 1: Overview of HTTP
Historical Perspective
http://www.w3.org/Protocols/WhyHTTP.html
This brief web page from 1991, from the author of HTTP, highlights some of the
original, minimalist goals of HTTP.
http://www.w3.org/History.html
“A Little History of the World Wide Web” gives a short but interesting perspec-
tive on some of the early goals and foundations of the World Wide Web and
HTTP.
http://www.w3.org/DesignIssues/Architecture.html
“Web Architecture from 50,000 Feet” paints a broad, ambitious view of the
World Wide Web and the design principles that affect HTTP and related web
technologies.
Other World Wide Web Information
http://www.w3.org
The World Wide Web Consortium (W3C) is the technology steering team for
the Web. The W3C develops interoperable technologies (specifications, guide-
lines, software, and tools) for the evolving Web. The W3C site is a treasure trove
of introductory and detailed documentation about web technologies.
http://www.ietf.org/rfc/rfc2396.txt
RFC 2396, “Uniform Resource Identifiers (URI): Generic Syntax,” is the detailed
reference for URIs and URLs.
http://www.ietf.org/rfc/rfc2141.txt
RFC 2141, “URN Syntax,” is a 1997 specification describing URN syntax.
http://www.ietf.org/rfc/rfc2046.txt
RFC 2046, “MIME Part 2: Media Types,” is the second in a suite of five Internet
specifications defining the Multipurpose Internet Mail Extensions standard for
multimedia content management.
http://www.wrec.org/Drafts/draft-ietf-wrec-taxonomy-06.txt
This Internet draft, “Internet Web Replication and Caching Taxonomy,” speci-
fies standard terminology for web architectural components.
23
CHAPTER 2
URLs and Resources
Think of the Internet as a giant, expanding city, full of places to see and things to do.
You and the other residents and tourists of this booming community would use stan-
dard naming conventions for the city’s vast attractions and services. You’d use street
addresses for museums, restaurants, and people’s homes. You’d use phone numbers
for the fire department, the boss’s secretary, and your mother, who says you don’t
call enough.
Everything has a standardized name, to help sort out the city’s resources. Books have
ISBN numbers, buses have route numbers, bank accounts have account numbers,
and people have social security numbers. Tomorrow you will meet your business
partners at gate 31 of the airport. Every morning you take a Red-line train and exit at
Kendall Square station.
And because everyone agreed on standards for these different names, we can easily
share the city’s treasures with each other. You can tell the cab driver to take you to
246 McAllister Street, and he’ll know what you mean (even if he takes the long way).
Uniform resource locators (URLs) are the standardized names for the Internet’s
resources. URLs point to pieces of electronic information, telling you where they are
located and how to interact with them.
In this chapter, we’ll cover:
• URL syntax and what the various URL components mean and do
• URL shortcuts that many web clients support, including relative URLs and
expandomatic URLs
• URL encoding and character rules
• Common URL schemes that support a variety of Internet information systems
• The future of URLs, including uniform resource names (URNs)—a framework
to support objects that move from place to place while retaining stable names
24 |Chapter 2: URLs and Resources
Navigating the Internet’s Resources
URLs are the resource locations that your browser needs to find information. They
let people and applications find, use, and share the billions of data resources on the
Internet. URLs are the usual human access point to HTTP and other protocols: a
person points a browser at a URL and, behind the scenes, the browser sends the
appropriate protocol messages to get the resource that the person wants.
URLs actually are a subset of a more general class of resource identifier called a uni-
form resource identifier, or URI. URIs are a general concept comprised of two main
subsets, URLs and URNs. URLs identify resources by describing where resources are
located, whereas URNs (which we’ll cover later in this chapter) identify resources by
name, regardless of where they currently reside.
The HTTP specification uses the more general concept of URIs as its resource identi-
fiers; in practice, however, HTTP applications deal only with the URL subset of
URIs. Throughout this book, we’ll sometimes refer to URIs and URLs interchange-
ably, but we’re almost always talking about URLs.
Say you want to fetch the URL http://www.joes-hardware.com/seasonal/index-fall.html:
• The first part of the URL (http) is the URL scheme. The scheme tells a web client
how to access the resource. In this case, the URL says to use the HTTP protocol.
• The second part of the URL (www.joes-hardware.com) is the server location.
This tells the web client where the resource is hosted.
• The third part of the URL (/seasonal/index-fall.html) is the resource path. The
path tells what particular local resource on the server is being requested.
See Figure 2-1 for an illustration.
URLs can direct you to resources available through protocols other than HTTP.
They can point you to any resource on the Internet, from a person’s email account:
mailto:president@whitehouse.gov
Figure 2-1. How URLs relate to browser, machine, server, and location on the server’s filesystem
http://www.joes-hardware.com/seasonal/index-fall.html
Client Server Disk
Scheme
(how)
Host
(where)
Path
(what)
Web page
index fall.html
Navigating the Internet’s Resources |25
to files that are available through other protocols, such as the File Transfer Protocol
(FTP):
ftp://ftp.lots-o-books.com/pub/complete-price-list.xls
to movies hosted off of streaming video servers:
rtsp://www.joes-hardware.com:554/interview/cto_video
URLs provide a way to uniformly name resources. Most URLs have the same
“scheme://server location/path” structure. So, for every resource out there and every
way to get those resources, you have a single way to name each resource so that any-
one can use that name to find it. However, this wasn’t always the case.
The Dark Days Before URLs
Before the Web and URLs, people relied on a rag-tag assortment of applications to
access data distributed throughout the Net. Most people were not lucky enough to
have all the right applications or were not savvy and patient enough to use them.
Before URLs came along, if you wanted to share the complete-catalog.xls file with a
friend, you would have had to say something like this: “Use FTP to connect to ftp.
joes-hardware.com. Log in as anonymous. Then type your username as the password.
Change to the pub directory. Switch to binary mode. Now download the file named
complete-catalog.xls to your local filesystem and view it there.”
Today, browsers such as Netscape Navigator and Microsoft Internet Explorer bun-
dle much of this functionality into one convenient package. Using URLs, these appli-
cations are able to access many resources in a uniform way, through one interface.
Instead of the complicated instructions above, you could just say “Point your
browser at ftp://ftp.lots-o-books.com/pub/complete-catalog.xls.”
URLs have provided a means for applications to be aware of how to access a
resource. In fact, many users are probably unaware of the protocols and access meth-
ods their browsers use to get the resources they are requesting.
With web browsers, you no longer need a news reader to read Internet news or an
FTP client to access files on FTP servers. You don’t need an electronic mail program
to send and receive email messages. URLs have helped to simplify the online world,
by allowing the browser to be smart about how to access and handle resources.*
Applications can use URLs to simplify access to information.
URLs give you and your browser all you need to find a piece of information. They
define the particular resource you want, where it is located, and how to get it.
* Browsers often use other applications to handle specific resources. For example, Internet Explorer launches
an email application to handle URLs that identify email resources.
26 |Chapter 2: URLs and Resources
URL Syntax
URLs provide a means of locating any resource on the Internet, but these resources
can be accessed by different schemes (e.g., HTTP, FTP, SMTP), and URL syntax var-
ies from scheme to scheme.
Does this mean that each different URL scheme has a radically different syntax? In
practice, no. Most URLs adhere to a general URL syntax, and there is significant
overlap in the style and syntax between different URL schemes.
Most URL schemes base their URL syntax on this nine-part general format:
<scheme>://<user>:<password>@<host>:<port>/<path>;<params>?<query>#<frag>
Almost no URLs contain all these components. The three most important parts of a
URL are the scheme, the host, and the path. Table 2-1 summarizes the various
components.
For example, consider the URL http://www.joes-hardware.com:80/index.html. The
scheme is “http”, the host is “www.joes-hardware.com”, the port is “80”, and the
path is “/index.html”.
Table 2-1. General URL components
Component Description Default value
scheme Which protocol to use when accessing a server to get a resource. None
user The username some schemes require to access a resource. anonymous
password The password that may be included after the username, separated by a colon (:). <Email address>
host The hostname or dotted IP address of the server hosting the resource. None
port The port number on which the server hosting the resource is listening. Many schemes
have default port numbers (the default port number for HTTP is 80).
Scheme-specific
path The local name for the resource on the server, separated from the previous URL com-
ponents by a slash (/). The syntax of the path component is server- and scheme-spe-
cific. (We will see later in this chapter that a URL’s path can be divided into segments,
and each segment can have its own components specific to that segment.)
None
params Used by some schemes to specify input parameters. Params are name/value pairs. A
URL can contain multiple params fields, separated from themselves and the rest of the
path by semicolons (;).
None
query Used by some schemes to pass parameters to active applications (such as databases,
bulletin boards, search engines, and other Internet gateways). There is no common
format for the contents of the query component. It is separated from the rest of the
URL by the “?” character.
None
frag A name for a piece or part of the resource. The frag field is not passed to the server
when referencing the object; it is used internally by the client. It is separated from the
rest of the URL by the “#” character.
None
URL Syntax |27
Schemes: What Protocol to Use
The scheme is really the main identifier of how to access a given resource; it tells the
application interpreting the URL what protocol it needs to speak. In our simple
HTTP URL, the scheme is simply “http”.
The scheme component must start with an alphabetic character, and it is separated
from the rest of the URL by the first “:” character. Scheme names are case-
insensitive, so the URLs “http://www.joes-hardware.com” and “HTTP://www.joes-
hardware.com” are equivalent.
Hosts and Ports
To find a resource on the Internet, an application needs to know what machine is
hosting the resource and where on that machine it can find the server that has access
to the desired resource. The host and port components of the URL provide these two
pieces of information.
The host component identifies the host machine on the Internet that has access to the
resource. The name can be provided as a hostname, as above (“www.joes-hardware.
com”) or as an IP address. For example, the following two URLs point to the same
resource—the first refers to the server by its hostname and the second by its IP address:
http://www.joes-hardware.com:80/index.html
http://161.58.228.45:80/index.html
The port component identifies the network port on which the server is listening. For
HTTP, which uses the underlying TCP protocol, the default port is 80.
Usernames and Passwords
More interesting components are the user and password components. Many servers
require a username and password before you can access data through them. FTP
servers are a common example of this. Here are a few examples:
ftp://ftp.prep.ai.mit.edu/pub/gnu
ftp://anonymous@ftp.prep.ai.mit.edu/pub/gnu
ftp://anonymous:my_passwd@ftp.prep.ai.mit.edu/pub/gnu
http://joe:joespasswd@www.joes-hardware.com/sales_info.txt
The first example has no user or password component, just our standard scheme,
host, and path. If an application is using a URL scheme that requires a username and
password, such as FTP, it generally will insert a default username and password if
they aren’t supplied. For example, if you hand your browser an FTP URL without
specifying a username and password, it will insert “anonymous” for your username
and send a default password (Internet Explorer sends “IEUser”, while Netscape Nav-
igator sends “mozilla”).
28 |Chapter 2: URLs and Resources
The second example shows a username being specified as “anonymous”. This user-
name, combined with the host component, looks just like an email address. The “@”
character separates the user and password components from the rest of the URL.
In the third example, both a username (“anonymous”) and password (“my_passwd”)
are specified, separated by the “:” character.
Paths
The path component of the URL specifies where on the server machine the resource
lives. The path often resembles a hierarchical filesystem path. For example:
http://www.joes-hardware.com:80/seasonal/index-fall.html
The path in this URL is “/seasonal/index-fall.html”, which resembles a filesystem
path on a Unix filesystem. The path is the information that the server needs to locate
the resource.*The path component for HTTP URLs can be divided into path seg-
ments separated by “/” characters (again, as in a file path on a Unix filesystem). Each
path segment can have its own params component.
Parameters
For many schemes, a simple host and path to the object just aren’t enough. Aside
from what port the server is listening to and even whether or not you have access to
the resource with a username and password, many protocols require more informa-
tion to work.
Applications interpreting URLs need these protocol parameters to access the
resource. Otherwise, the server on the other side might not service the request or,
worse yet, might service it wrong. For example, take a protocol like FTP, which has
two modes of transfer, binary and text. You wouldn’t want your binary image trans-
ferred in text mode, because the binary image could be scrambled.
To give applications the input parameters they need in order to talk to the server cor-
rectly, URLs have a params component. This component is just a list of name/value
pairs in the URL, separated from the rest of the URL (and from each other) by “;”
characters. They provide applications with any additional information that they need
to access the resource. For example:
ftp://prep.ai.mit.edu/pub/gnu;type=d
In this example, there is one param, type=d, where the name of the param is “type”
and its value is “d”.
* This is a bit of a simplification. In “Virtual Hosting” in Chapter 18, we will see that the path is not always
enough information to locate a resource. Sometimes a server needs additional information.
URL Syntax |29
As we mentioned earlier, the path component for HTTP URLs can be broken into
path segments. Each segment can have its own params. For example:
http://www.joes-hardware.com/hammers;sale=false/index.html;graphics=true
In this example there are two path segments, hammers and index.html. The hammers
path segment has the param sale, and its value is false. The index.html segment has
the param graphics, and its value is true.
Query Strings
Some resources, such as database services, can be asked questions or queries to nar-
row down the type of resource being requested.
Let’s say Joe’s Hardware store maintains a list of unsold inventory in a database and
allows the inventory to be queried, to see whether products are in stock. The follow-
ing URL might be used to query a web database gateway to see if item number 12731
is available:
http://www.joes-hardware.com/inventory-check.cgi?item=12731
For the most part, this resembles the other URLs we have looked at. What is new is
everything to the right of the question mark (?). This is called the query component.
The query component of the URL is passed along to a gateway resource, with the
path component of the URL identifying the gateway resource. Basically, gateways
can be thought of as access points to other applications (we discuss gateways in
detail in Chapter 8).
Figure 2-2 shows an example of a query component being passed to a server that is
acting as a gateway to Joe’s Hardware’s inventory-checking application. The query is
checking whether a particular item, 12731, is in inventory in size large and color
blue.
There is no requirement for the format of the query component, except that some
characters are illegal, as we’ll see later in this chapter. By convention, many gateways
Figure 2-2. The URL query component is sent along to the gateway application
http://www.joes-hardware.com/inventory-check.cgi?item=12731&color=blue&size=large
Client Server
Internet
item=12731&color=blue&size=large
“Inventory-check”
gateway
30 |Chapter 2: URLs and Resources
expect the query string to be formatted as a series of “name=value” pairs, separated
by “&” characters:
http://www.joes-hardware.com/inventory-check.cgi?item=12731&color=blue
In this example, there are two name/value pairs in the query component: item=12731
and color=blue.
Fragments
Some resource types, such as HTML, can be divided further than just the resource
level. For example, for a single, large text document with sections in it, the URL for
the resource would point to the entire text document, but ideally you could specify
the sections within the resource.
To allow referencing of parts or fragments of a resource, URLs support a frag com-
ponent to identify pieces within a resource. For example, a URL could point to a par-
ticular image or section within an HTML document.
A fragment dangles off the right-hand side of a URL, preceded by a #character. For
example:
http://www.joes-hardware.com/tools.html#drills
In this example, the fragment drills references a portion of the /tools.html web page
located on the Joe’s Hardware web server. The portion is named “drills”.
Because HTTP servers generally deal only with entire objects,*not with fragments of
objects, clients don’t pass fragments along to servers (see Figure 2-3). After your
browser gets the entire resource from the server, it then uses the fragment to display
the part of the resource in which you are interested.
URL Shortcuts
Web clients understand and use a few URL shortcuts. Relative URLs are a convenient
shorthand for specifying a resource within a resource. Many browsers also support
“automatic expansion” of URLs, where the user can type in a key (memorable) part of
a URL, and the browser fills in the rest. This is explained in “Expandomatic URLs.”
Relative URLs
URLs come in two flavors: absolute and relative. So far, we have looked only at abso-
lute URLs. With an absolute URL, you have all the information you need to access a
resource.
* In “Range Requests” in Chapter 15, we will see that HTTP agents may request byte ranges of objects. How-
ever, in the context of URL fragments, the server sends the entire object and the agent applies the fragment
identifier to the resource.
URL Shortcuts |31
On the other hand, relative URLs are incomplete. To get all the information needed
to access a resource from a relative URL, you must interpret it relative to another
URL, called its base.
Relative URLs are a convenient shorthand notation for URLs. If you have ever writ-
ten HTML by hand, you have probably found them to be a great shortcut.
Example 2-1 contains an example HTML document with an embedded relative URL.
In Example 2-1, we have an HTML document for the resource:
http://www.joes-hardware.com/tools.html
In the HTML document, there is a hyperlink containing the URL ./hammers.html.
This URL seems incomplete, but it is a legal relative URL. It can be interpreted rela-
tive to the URL of the document in which it is found; in this case, relative to the
resource /tools.html on the Joe’s Hardware web server.
Figure 2-3. The URL fragment is used only by the client, because the server deals with entire objects
Example 2-1. HTML snippet with relative URLs
<HTML>
<HEAD><TITLE>Joe's Tools</TITLE></HEAD>
<BODY>
<H1> Tools Page </H1>
<H2> Hammers <H2>
<P> Joe's Hardware Online has the largest selection of <A HREF="./hammers.html">hammers
</A> on earth.
</BODY>
</HTML>
Client www.joes-hardware.com
Internet
(b) Browser makes request to http://www.joes-hardware.com/tools.html
(a) User selects link to “http://www.joes-hardware.com/tools.html#drills “
http://www.joes-hardware.com/tools.html#drills
(d) Browser displays HTML page starting with
named “drills” fragment
(c) Server returns entire HTML page
Browser scrolls down to start
at named “drills” fragment
(Fragment is NOT sent to the server)
32 |Chapter 2: URLs and Resources
The abbreviated relative URL syntax lets HTML authors omit from URLs the
scheme, host, and other components. These components can be inferred by the base
URL of the resource they are in. URLs for other resources also can be specified in this
shorthand.
In Example 2-1, our base URL is:
http://www.joes-hardware.com/tools.html
Using this URL as a base, we can infer the missing information. We know the
resource is ./hammers.html, but we don’t know the scheme or host. Using the base
URL, we can infer that the scheme is http and the host is www.joes-hardware.com.
Figure 2-4 illustrates this.
Relative URLs are only fragments or pieces of URLs. Applications that process URLs
(such as your browser) need to be able to convert between relative and absolute
URLs.
It is also worth noting that relative URLs provide a convenient way to keep a set of
resources (such as HTML pages) portable. If you use relative URLs, you can move a
set of documents around and still have their links work, because they will be inter-
preted relative to the new base. This allows for things like mirroring content on other
servers.
Base URLs
The first step in the conversion process is to find a base URL. The base URL serves as
a point of reference for the relative URL. It can come from a few places:
Explicitly provided in the resource
Some resources explicitly specify the base URL. An HTML document, for exam-
ple, may include a <BASE> HTML tag defining the base URL by which to convert
all relative URLs in that HTML document.
Base URL of the encapsulating resource
If a relative URL is found in a resource that does not explicitly specify a base
URL, as in Example 2-1, it can use the URL of the resource in which it is embed-
ded as a base (as we did in our example).
Figure 2-4. Using a base URL
http://www.joes-hardware.com/tools.html
Base URL:
./hammers.html
Relative URL:
http://www.joes-hardware.com/hammers.html
New absolute URL
URL Shortcuts |33
No base URL
In some instances, there is no base URL. This often means that you have an
absolute URL; however, sometimes you may just have an incomplete or broken
URL.
Resolving relative references
Previously, we showed the basic components and syntax of URLs. The next step in
converting a relative URL into an absolute one is to break up both the relative and
base URLs into their component pieces.
In effect, you are just parsing the URL, but this is often called decomposing the URL,
because you are breaking it up into its components. Once you have broken the base
and relative URLs into their components, you can then apply the algorithm pictured
in Figure 2-5 to finish the conversion.
Figure 2-5. Converting relative to absolute URLs
Parsed relative URL:
{ scheme= X, user= Y, . . . }
Inherit base URL scheme
Examine user, password,
host, and port components
Inherit base URL, user, password,
host, and port
Examine path component
Inherit base URL path
Examine param component Have absolute path proceed
Remove “./” and “<seg>/./” from path
Proceed
Inherit base URL param
Examine query component
Inherit base URL query
Proceed
Combine inherited and relative components into new absolute URL
Defaults to base URL is absolute
Scheme empty All components empty Nonempty scheme
All components empty
Path empty Nonempty path
with leading “/”
Nonempty path
w/o leading “/”
Param
empty
Query empty Query nonempty
Param
nonempty
34 |Chapter 2: URLs and Resources
This algorithm converts a relative URL to its absolute form, which can then be used
to reference the resource. This algorithm was originally specified in RFC 1808 and
later incorporated into RFC 2396.
With our ./hammers.html example from Example 2-1, we can apply the algorithm
depicted in Figure 2-5:
1. Path is ./hammers.html; base URL is http://www.joes-hardware.com/tools.html.
2. Scheme is empty; proceed down left half of chart and inherit the base URL
scheme (HTTP).
3. At least one component is non-empty; proceed to bottom, inheriting host and
port components.
4. Combining the components we have from the relative URL (path: ./hammers.html)
with what we have inherited (scheme: http, host: www.joes-hardware.com, port:
80), we get our new absolute URL: http://www.joes-hardware.com/hammers.html.
Expandomatic URLs
Some browsers try to expand URLs automatically, either after you submit the URL
or while you’re typing. This provides users with a shortcut: they don’t have to type in
the complete URL, because it automatically expands itself.
These “expandomatic” features come in two flavors:
Hostname expansion
In hostname expansion, the browser can often expand the hostname you type in
into the full hostname without your help, just by using some simple heuristics.
For example if you type “yahoo” in the address box, your browser can automati-
cally insert “www.” and “.com” onto the hostname, creating “www.yahoo.com”.
Some browsers will try this if they are unable to find a site that matches “yahoo”,
trying a few expansions before giving up. Browsers apply these simple tricks to
save you some time and frustration.
However, these expansion tricks on hostnames can cause problems for other
HTTP applications, such as proxies. In Chapter 6, we will discuss these prob-
lems in more detail.
History expansion
Another technique that browsers use to save you time typing URLs is to store a
history of the URLs that you have visited in the past. As you type in the URL,
they can offer you completed choices to select from by matching what you type
to the prefixes of the URLs in your history. So, if you were typing in the start of a
URL that you had visited previously, such as http://www.joes-, your browser
could suggest http://www.joes-hardware.com. You could then select that instead
of typing out the complete URL.
Shady Characters |35
Be aware that URL auto-expansion may behave differently when used with proxies.
We discuss this further in “URI Client Auto-Expansion and Hostname Resolution”
in Chapter 6.
Shady Characters
URLs were designed to be portable. They were also designed to uniformly name all
the resources on the Internet, which means that they will be transmitted through
various protocols. Because all of these protocols have different mechanisms for
transmitting their data, it was important for URLs to be designed so that they could
be transmitted safely through any Internet protocol.
Safe transmission means that URLs can be transmitted without the risk of losing
information. Some protocols, such as the Simple Mail Transfer Protocol (SMTP) for
electronic mail, use transmission methods that can strip off certain characters.*To
get around this, URLs are permitted to contain only characters from a relatively
small, universally safe alphabet.
In addition to wanting URLs to be transportable by all Internet protocols, designers
wanted them to be readable by people. So invisible, nonprinting characters also are
prohibited in URLs, even though these characters may pass through mailers and oth-
erwise be portable.†
To complicate matters further, URLs also need to be complete. URL designers real-
ized there would be times when people would want URLs to contain binary data or
characters outside of the universally safe alphabet. So, an escape mechanism was
added, allowing unsafe characters to be encoded into safe characters for transport.
This section summarizes the universal alphabet and encoding rules for URLs.
The URL Character Set
Default computer system character sets often have an Anglocentric bias. Histori-
cally, many computer applications have used the US-ASCII character set. US-ASCII
uses 7 bits to represent most keys available on an English typewriter and a few non-
printing control characters for text formatting and hardware signalling.
US-ASCII is very portable, due to its long legacy. But while it’s convenient to citizens of
the U.S., it doesn’t support the inflected characters common in European languages or
the hundreds of non-Romanic languages read by billions of people around the world.
* This is caused by the use of a 7-bit encoding for messages; this can strip off information if the source is
encoded in 8 bits or more.
† Nonprinting characters include whitespace (note that RFC 2396 recommends that applications ignore
whitespace).
36 |Chapter 2: URLs and Resources
Furthermore, some URLs may need to contain arbitrary binary data. Recognizing the
need for completeness, the URL designers have incorporated escape sequences.
Escape sequences allow the encoding of arbitrary character values or data using a
restricted subset of the US-ASCII character set, yielding portability and completeness.
Encoding Mechanisms
To get around the limitations of a safe character set representation, an encoding
scheme was devised to represent characters in a URL that are not safe. The encoding
simply represents the unsafe character by an “escape” notation, consisting of a per-
cent sign (%) followed by two hexadecimal digits that represent the ASCII code of
the character.
Table 2-2 shows a few examples.
Character Restrictions
Several characters have been reserved to have special meaning inside of a URL. Oth-
ers are not in the defined US-ASCII printable set. And still others are known to con-
fuse some Internet gateways and protocols, so their use is discouraged.
Table 2-3 lists characters that should be encoded in a URL before you use them for
anything other than their reserved purposes.
Table 2-2. Some encoded character examples
Character ASCII code Example URL
~ 126 (0x7E) http://www.joes-hardware.com/%7Ejoe
SPACE 32 (0x20) http://www.joes-hardware.com/more%20tools.html
% 37 (0x25) http://www.joes-hardware.com/100%25satisfaction html
Table 2-3. Reserved and restricted characters
Character Reservation/Restriction
% Reserved as escape token for encoded characters
/ Reserved for delimiting splitting up path segments in the path component
. Reserved in the path component
.. Reserved in the path component
# Reserved as the fragment delimiter
? Reserved as the query-string delimiter
; Reserved as the params delimiter
: Reserved to delimit the scheme, user/password, and host/port components
$ , + Reserved
@ & = Reserved because they have special meaning in the context of some schemes
Shady Characters |37
A Bit More
You might be wondering why nothing bad has happened when you have used char-
acters that are unsafe. For instance, you can visit Joe’s home page at:
http://www.joes-hardware.com/~joe
and not encode the “~” character. For some transport protocols this is not an issue,
but it is still unwise for application developers not to encode unsafe characters.
Applications need to walk a fine line. It is best for client applications to convert any
unsafe or restricted characters before sending any URL to any other application.*
Once all the unsafe characters have been encoded, the URL is in a canonical form
that can be shared between applications; there is no need to worry about the other
application getting confused by any of the characters’ special meanings.
The original application that gets the URL from the user is best fit to determine
which characters need to be encoded. Because each component of the URL may have
its own safe/unsafe characters, and which characters are safe/unsafe is scheme-
dependent, only the application receiving the URL from the user really is in a posi-
tion to determine what needs to be encoded.
Of course, the other extreme is for the application to encode all characters. While this
is not recommended, there is no hard and fast rule against encoding characters that are
considered safe already; however, in practice this can lead to odd and broken behav-
ior, because some applications may assume that safe characters will not be encoded.
Sometimes, malicious folks encode extra characters in an attempt to get around
applications that are doing pattern matching on URLs—for example, web filtering
applications. Encoding safe URL components can cause pattern-matching applica-
tions to fail to recognize the patterns for which they are searching. In general, appli-
cations interpreting URLs must decode the URLs before processing them.
{ } | \ ^ ~ [ ] ‘Restricted because of unsafe handling by various transport agents, such as gateways
< > " Unsafe; should be encoded because these characters often have meaning outside the scope of the URL,
such as delimiting the URL itself in a document (e.g., “http://www.joes-hardware.com”)
0x00–0x1F, 0x7F Restricted; characters within thesehex ranges fallwithin the nonprintable section ofthe US-ASCII charac-
ter set
> 0x7F Restricted; characters whose hex values fall within this range do not fall within the 7-bit range of the US-
ASCII character set
* Here we are specifically talking about client applications, not other HTTP intermediaries, like proxies. In
“In-Flight URI Modification” in Chapter 6, we discuss some of the problems that can arise when proxies or
other intermediary HTTP applications attempt to change (e.g., encode) URLs on the behalf of a client.
Table 2-3. Reserved and restricted characters (continued)
Character Reservation/Restriction
38 |Chapter 2: URLs and Resources
Some URL components, such as the scheme, need to be recognized readily and are
required to start with an alphabetic character. Refer back to “URL Syntax” for more
guidelines on the use of reserved and unsafe characters within different URL
components.*
A Sea of Schemes
In this section, we’ll take a look at the more common scheme formats on the Web.
Appendix A gives a fairly exhaustive list of schemes and references to their individ-
ual documentation.
Table 2-4 summarizes some of the most popular schemes. Reviewing “URL Syntax”
will make the syntax portion of the table a little more familiar.
* Table 2-3 lists reserved characters for the various URL components. In general, encoding should be limited
to those characters that are unsafe for transport.
Table 2-4. Common scheme formats
Scheme Description
http The Hypertext Transfer Protocol scheme conforms to the general URL format, except that there is no username
or password. The port defaults to 80 if omitted.
Basic form:
http://<host>:<port>/<path>?<query>#<frag>
Examples:
http://www.joes-hardware.com/index.html
http://www.joes-hardware.com:80/index html
https The https scheme is a twin to the http scheme. The only difference is that the https scheme uses Netscape’s
Secure Sockets Layer (SSL), which provides end-to-end encryption of HTTP connections. Its syntax is identical to
that of HTTP, with a default port of 443.
Basic form:
https://<host>:<port>/<path>?<query>#<frag>
Example:
https://www.joes-hardware.com/secure.html
mailto Mailto URLs refer to email addresses. Because email behaves differently from other schemes (it does not refer to
objects that can be accessed directly), the format of a mailto URL differs from that of the standard URL. The syn-
tax for Internet email addresses is documented in Internet RFC 822.
Basic form:
mailto:<RFC-822-addr-spec>
Example:
mailto:joe@joes-hardware.com
A Sea of Schemes |39
ftp File Transfer Protocol URLs can be used to download and upload files on an FTP server and to obtain listings of
the contents of a directory structure on an FTP server.
FTP has been around since before the advent of the Web and URLs. Web applications have assimilated FTP as a
data-access scheme. The URL syntax follows the general form.
Basic form:
ftp //<user>:<password>@<host>:<port>/<path>;<params>
Example:
ftp //anonymous:joe%40joes-hardware.com@prep.ai.mit.edu:21/pub/gnu/
rtsp, rtspu RTSP URLs are identifiers for audio and video media resources that can be retrieved through the Real Time
Streaming Protocol.
The “u” in the rtspu scheme denotes that the UDP protocol is used to retrieve the resource.
Basic forms:
rtsp://<user>:<password>@<host>:<port>/<path>
rtspu://<user>:<password>@<host>:<port>/<path>
Example:
rtsp://www.joes-hardware.com:554/interview/cto_video
file The file scheme denotes files directly accessible on a given host machine (by local disk, a network filesystem, or
some other file-sharing system). The fields follow the general form. If the host is omitted, it defaults to the local
host from which the URL is being used.
Basic form:
file://<host>/<path>
Example:
file://OFFICE-FS/policies/casual-fridays.doc
news The news scheme is used to access specific articles or newsgroups, as defined by RFC 1036. It has the unusual
property that a news URL in itself does not contain sufficient information to locate the resource.
The news URL is missing information about where to acquire the resource—no hostname or machine name is
supplied. It is the interpreting application’s job to acquire this information from the user. For example, in your
Netscape browser, under the Options menu, you can specify your NNTP (news) server. This tells your browser
what server to use when it has a news URL.
News resources can be accessed from multiple servers. They are said to be location-independent, as they are not
dependent on any one source for access.
The “@” character is reserved within a news URL and is used to distinguish between news URLs that refer to
newsgroups and news URLs that refer to specific news articles.
Basic forms:
news:<newsgroup>
news:<news-article-id>
Example:
news:rec.arts startrek
Table 2-4. Common scheme formats (continued)
Scheme Description
40 |Chapter 2: URLs and Resources
The Future
URLs are a powerful tool. Their design allows them to name all existing objects and
easily encompass new formats. They provide a uniform naming mechanism that can
be shared between Internet protocols.
However, they are not perfect. URLs are really addresses, not true names. This
means that a URL tells you where something is located, for the moment. It provides
you with the name of a specific server on a specific port, where you can find the
resource. The downfall of this scheme is that if the resource is moved, the URL is no
longer valid. And at that point, it provides no way to locate the object.
What would be ideal is if you had the real name of an object, which you could use to
look up that object regardless of its location. As with a person, given the name of the
resource and a few other facts, you could track down that resource, regardless of
where it moved.
The Internet Engineering Task Force (IETF) has been working on a new standard,
uniform resource names (URNs), for some time now, to address just this issue.
URNs provide a stable name for an object, regardless of where that object moves
(either inside a web server or across web servers).
Persistent uniform resource locators (PURLs) are an example of how URN functional-
ity can be achieved using URLs. The concept is to introduce another level of indirec-
tion in looking up a resource, using an intermediary resource locator server that
catalogues and tracks the actual URL of a resource. A client can request a persistent
URL from the locator, which can then respond with a resource that redirects the cli-
ent to the actual and current URL for the resource (see Figure 2-6). For more infor-
mation on PURLs, visit http://purl.oclc.org.
If Not Now, When?
The ideas behind URNs have been around for some time. Indeed, if you look at the
publication dates for some of their specifications, you might ask yourself why they
have yet to be adopted.
telnet The telnet scheme is used to access interactive services. It does not represent an object per se, but an interactive
application (resource) accessible via the telnet protocol.
Basic form:
telnet //<user>:<password>@<host>:<port>/
Example:
telnet //slurp:webhound@joes-hardware.com:23/
Table 2-4. Common scheme formats (continued)
Scheme Description
For More Information |41
The change from URLs to URNs is an enormous task. Standardization is a slow pro-
cess, often for good reason. Support for URNs will require many changes—consensus
from the standards bodies, modifications to various HTTP applications, etc. A tre-
mendous amount of critical mass is required to make such changes, and unfortu-
nately (or perhaps fortunately), there is so much momentum behind URLs that it will
be some time before all the stars align to make such a conversion possible.
Throughout the explosive growth of the Web, Internet users—everyone from com-
puter scientists to the average Internet user—have been taught to use URLs. While
they suffer from clumsy syntax (for the novice) and persistence problems, people have
learned how to use them and how to deal with their drawbacks. URLs have some lim-
itations, but they’re not the web development community’s most pressing problem.
Currently, and for the foreseeable future, URLs are the way to name resources on the
Internet. They are everywhere, and they have proven to be a very important part of
the Web’s success. It will be a while before any other naming scheme unseats URLs.
However, URLs do have their limitations, and it is likely that new standards (possi-
bly URNs) will emerge and be deployed to address some of these limitations.
For More Information
For more information on URLs, refer to:
http://www.w3.org/Addressing/
The W3C web page about naming and addressing URIs and URLs.
http://www.ietf.org/rfc/rfc1738
RFC 1738, “Uniform Resource Locators (URL),” by T. Berners-Lee, L. Masinter,
and M. McCahill.
Figure 2-6. PURLs use a resource locator server to name the current location of a resource
Client purl.oclc.org
Internet
Get http://purl.oclc.org/jhardware/
STEP 1: Ask the resource resolver what the
Joe’s Hardware URL is. Receive from the
resolver the current location of the resource.
Actual: http://www.joes-hardware.com/
STEP 2: Get the actual URL for the resource
Client www.joes-hardware.com
Internet
Get http://www.joes-hardware.com/
42 |Chapter 2: URLs and Resources
http://www.ietf.org/rfc/rfc2396.txt
RFC 2396, “Uniform Resource Identifiers (URI): Generic Syntax,” by T. Berners-
Lee, R. Fielding, and L. Masinter.
http://www.ietf.org/rfc/rfc2141.txt
RFC 2141, “URN Syntax,” by R. Moats.
http://purl.oclc.org
The persistent uniform resource locator web site.
http://www.ietf.org/rfc/rfc1808.txt
RFC 1808, “Relative Uniform Resource Locators,” by R. Fielding.
43
CHAPTER 3
HTTP Messages
If HTTP is the Internet’s courier, HTTP messages are the packages it uses to move
things around. In Chapter 1, we showed how HTTP programs send each other mes-
sages to get work done. This chapter tells you all about HTTP messages—how to
create them and how to understand them. After reading this chapter, you’ll know
most of what you need to know to write your own HTTP applications. In particular,
you’ll understand:
• How messages flow
• The three parts of HTTP messages (start line, headers, and entity body)
• The differences between request and response messages
• The various functions (methods) that request messages support
• The various status codes that are returned with response messages
• What the various HTTP headers do
The Flow of Messages
HTTP messages are the blocks of data sent between HTTP applications. These
blocks of data begin with some text meta-information describing the message con-
tents and meaning, followed by optional data. These messages flow between clients,
servers, and proxies. The terms “inbound,” “outbound,” “upstream,” and “down-
stream” describe message direction.
Messages Commute Inbound to the Origin Server
HTTP uses the terms inbound and outbound to describe transactional direction. Mes-
sages travel inbound to the origin server, and when their work is done, they travel
outbound back to the user agent (see Figure 3-1).
44 |Chapter 3: HTTP Messages
Messages Flow Downstream
HTTP messages flow like rivers. All messages flow downstream, regardless of whether
they are request messages or response messages (see Figure 3-2). The sender of any
message is upstream of the receiver. In Figure 3-2, proxy 1 is upstream of proxy 3 for
the request but downstream of proxy 3 for the response.*
The Parts of a Message
HTTP messages are simple, formatted blocks of data. Take a peek at Figure 3-3 for
an example. Each message contains either a request from a client or a response from
a server. They consist of three parts: a start line describing the message, a block of
headers containing attributes, and an optional body containing data.
The start line and headers are just ASCII text, broken up by lines. Each line ends with
a two-character end-of-line sequence, consisting of a carriage return (ASCII 13) and a
line-feed character (ASCII 10). This end-of-line sequence is written “CRLF.” It is
worth pointing out that while the HTTP specification for terminating lines is CRLF,
robust applications also should accept just a line-feed character. Some older or bro-
ken HTTP applications do not always send both the carriage return and line feed.
The entity body or message body (or just plain “body”) is simply an optional chunk
of data. Unlike the start line and headers, the body can contain text or binary data or
can be empty.
In the example in Figure 3-3, the headers give you a bit of information about the
body. The Content-Type line tells you what the body is—in this example, it is a
plain-text document. The Content-Length line tells you how big the body is; here it
is a meager 19 bytes.
Figure 3-1. Messages travel inbound to the origin server and outbound back to the client
* The terms “upstream” and “downstream” relate only to the sender and receiver. We can’t tell whether a mes-
sage is heading to the origin server or the client, because both are downstream.
ServerClient
Proxy 1
Inbound (to server) GET /index.html HTTP/1.0
Outbound (to user agent)
HTTP/1.0 200 OK
Content-type: text/html
...
Proxy 2 Proxy 3
The Parts of a Message |45
Message Syntax
All HTTP messages fall into two types: request messages and response messages.
Request messages request an action from a web server. Response messages carry
results of a request back to a client. Both request and response messages have the
same basic message structure. Figure 3-4 shows request and response messages to get
a GIF image.
Here’s the format for a request message:
<method> <request-URL> <version>
<headers>
<entity-body>
Figure 3-2. All messages flow downstream
Figure 3-3. Three parts of an HTTP message
Client Proxy 1
Proxy 3
Proxy 2
Proxy 1
Client
Server
Request (flowing downstream)
Response (flowing downstream)
No messages ever go upstream
Proxy 2
Proxy 3
HTTP/1.0 200 OK
Content-type: text/plain
Content-length: 19
Hi! I’m a message!
Start line
Headers
Body
Client Server
46 |Chapter 3: HTTP Messages
Here’s the format for a response message (note that the syntax differs only in the
start line):
<version> <status> <reason-phrase>
<headers>
<entity-body>
Here’s a quick description of the various parts:
method
The action that the client wants the server to perform on the resource. It is a sin-
gle word, like “GET,” “HEAD,” or “POST”. We cover the method in detail later
in this chapter.
request-URL
A complete URL naming the requested resource, or the path component of the
URL. If you are talking directly to the server, the path component of the URL is
usually okay as long as it is the absolute path to the resource—the server can
assume itself as the host/port of the URL. Chapter 2 covers URL syntax in detail.
version
The version of HTTP that the message is using. Its format looks like:
HTTP/<major>.<minor>
where major and minor both are integers. We discuss HTTP versioning a bit
more later in this chapter.
status-code
A three-digit number describing what happened during the request. The first
digit of each code describes the general class of status (“success,” “error,” etc.).
An exhaustive list of status codes defined in the HTTP specification and their
meanings is provided later in this chapter.
Figure 3-4. An HTTP transaction has request and response messages
Internet
HTTP request message contains
the command and the URL
GET /specials/saw-blade.gif HTTP/1.0
Host: www.joes-hardware.com
Client www.joes-hardware.com
HTTP/1.0 200 OK
Content-Type: image/gif
Content-Length: 8572 HTTP response message contains
the result of the transaction
The Parts of a Message |47
reason-phrase
A human-readable version of the numeric status code, consisting of all the text
until the end-of-line sequence. Example reason phrases for all the status codes
defined in the HTTP specification are provided later in this chapter. The reason
phrase is meant solely for human consumption, so, for example, response lines
containing “HTTP/1.0 200 NOT OK” and “HTTP/1.0 200 OK” should be
treated as equivalent success indications, despite the reason phrases suggesting
otherwise.
headers
Zero or more headers, each of which is a name, followed by a colon (:), fol-
lowed by optional whitespace, followed by a value, followed by a CRLF. The
headers are terminated by a blank line (CRLF), marking the end of the list of
headers and the beginning of the entity body. Some versions of HTTP, such as
HTTP/1.1, require certain headers to be present for the request or response mes-
sage to be valid. The various HTTP headers are covered later in this chapter.
entity-body
The entity body contains a block of arbitrary data. Not all messages contain
entity bodies, so sometimes a message terminates with a bare CRLF. We discuss
entities in detail in Chapter 15.
Figure 3-5 demonstrates hypothetical request and response messages.
Note that a set of HTTP headers should always end in a blank line (bare CRLF), even
if there are no headers and even if there is no entity body. Historically, however,
many clients and servers (mistakenly) omitted the final CRLF if there was no entity
body. To interoperate with these popular but noncompliant implementations, cli-
ents and servers should accept messages that end without the final CRLF.
Start Lines
All HTTP messages begin with a start line. The start line for a request message says
what to do. The start line for a response message says what happened.
Figure 3-5. Example request and response messages
GET /test/hi-there.txt HTTP/1.1
Accept: text/*
Host: www.joes-hardware.com
HTTP/1.0 200 OK
Content-type: text/plain
Content-length: 19
Hi! I’m a message!
Start line
Headers
Body
(a) Request message (b) Response message
48 |Chapter 3: HTTP Messages
Request line
Request messages ask servers to do something to a resource. The start line for a
request message, or request line, contains a method describing what operation the
server should perform and a request URL describing the resource on which to per-
form the method. The request line also includes an HTTP version which tells the
server what dialect of HTTP the client is speaking.
All of these fields are separated by whitespace. In Figure 3-5a, the request method is
GET, the request URL is /test/hi-there.txt, and the version is HTTP/1.1. Prior to
HTTP/1.0, request lines were not required to contain an HTTP version.
Response line
Response messages carry status information and any resulting data from an opera-
tion back to a client. The start line for a response message, or response line, contains
the HTTP version that the response message is using, a numeric status code, and a
textual reason phrase describing the status of the operation.
All these fields are separated by whitespace. In Figure 3-5b, the HTTP version is
HTTP/1.0, the status code is 200 (indicating success), and the reason phrase is OK,
meaning the document was returned successfully. Prior to HTTP/1.0, responses were
not required to contain a response line.
Methods
The method begins the start line of requests, telling the server what to do. For exam-
ple, in the line “GET /specials/saw-blade.gif HTTP/1.0,” the method is GET.
The HTTP specifications have defined a set of common request methods. For exam-
ple, the GET method gets a document from a server, the POST method sends data to
a server for processing, and the OPTIONS method determines the general capabili-
ties of a web server or the capabilities of a web server for a specific resource.
Table 3-1 describes seven of these methods. Note that some methods have a body in
the request message, and other methods have bodyless requests.
Table 3-1. Common HTTP methods
Method Description Message body?
GET Get a document from the server. No
HEAD Get just the headers for a document from the server. No
POST Send data to the server for processing. Yes
PUT Store the body of the request on the server. Yes
TRACE Trace the message through proxy servers to the server. No
OPTIONS Determine what methods can operate on a server. No
DELETE Remove a document from the server. No
The Parts of a Message |49
Not all servers implement all seven of the methods in Table 3-1. Furthermore,
because HTTP was designed to be easily extensible, other servers may implement
their own request methods in addition to these. These additional methods are called
extension methods, because they extend the HTTP specification.
Status codes
As methods tell the server what to do, status codes tell the client what happened.
Status codes live in the start lines of responses. For example, in the line “HTTP/1.0
200 OK,” the status code is 200.
When clients send request messages to an HTTP server, many things can happen. If
you are fortunate, the request will complete successfully. You might not always be so
lucky. The server may tell you that the resource you requested could not be found,
that you don’t have permission to access the resource, or perhaps that the resource
has moved someplace else.
Status codes are returned in the start line of each response message. Both a numeric
and a human-readable status are returned. The numeric code makes error process-
ing easy for programs, while the reason phrase is easily understood by humans.
The different status codes are grouped into classes by their three-digit numeric codes.
Status codes between 200 and 299 represent success. Codes between 300 and 399
indicate that the resource has been moved. Codes between 400 and 499 mean that
the client did something wrong in the request. Codes between 500 and 599 mean
something went awry on the server.
The status code classes are shown in Table 3-2.
Current versions of HTTP define only a few codes for each status category. As the
protocol evolves, more status codes will be defined officially in the HTTP specifica-
tion. If you receive a status code that you don’t recognize, chances are someone has
defined it as an extension to the current protocol. You should treat it as a general
member of the class whose range it falls into.
For example, if you receive status code 515 (which is outside of the defined range for
5XX codes listed in Table 3-2), you should treat the response as indicating a server
error, which is the general class of 5XX messages.
Table 3-2. Status code classes
Overall range Defined range Category
100-199 100-101 Informational
200-299 200-206 Successful
300-399 300-305 Redirection
400-499 400-415 Client error
500-599 500-505 Server error
50 |Chapter 3: HTTP Messages
Table 3-3 lists some of the most common status codes that you will see. We will
explain all the current HTTP status codes in detail later in this chapter.
Reason phrases
The reason phrase is the last component of the start line of the response. It provides
a textual explanation of the status code. For example, in the line “HTTP/1.0 200
OK,” the reason phrase is OK.
Reason phrases are paired one-to-one with status codes. The reason phrase provides
a human-readable version of the status code that application developers can pass
along to their users to indicate what happened during the request.
The HTTP specification does not provide any hard and fast rules for what reason
phrases should look like. Later in this chapter, we list the status codes and some sug-
gested reason phrases.
Version numbers
Version numbers appear in both request and response message start lines in the for-
mat HTTP/x.y. They provide a means for HTTP applications to tell each other what
version of the protocol they conform to.
Version numbers are intended to provide applications speaking HTTP with a clue
about each other’s capabilities and the format of the message. An HTTP Version 1.2
application communicating with an HTTP Version 1.1 application should know that
it should not use any new 1.2 features, as they likely are not implemented by the
application speaking the older version of the protocol.
The version number indicates the highest version of HTTP that an application sup-
ports. In some cases this leads to confusion between applications,*because HTTP/1.0
applications interpret a response with HTTP/1.1 in it to indicate that the response is
a 1.1 response, when in fact that’s just the level of protocol used by the responding
application.
Note that version numbers are not treated as fractional numbers. Each number in the
version (for example, the “1” and “0” in HTTP/1.0) is treated as a separate number.
So, when comparing HTTP versions, each number must be compared separately in
Table 3-3. Common status codes
Status code Reason phrase Meaning
200 OK Success! Any requested data is in the response body.
401 Unauthorized You need to enter a username and password.
404 Not Found The server cannot find a resource for the requested URL.
* See http://httpd.apache.org/docs-2.0/misc/known_client_problems.html for more on cases in which Apache
has run into this problem with clients.
The Parts of a Message |51
order to determine which is the higher version. For example, HTTP/2.22 is a higher
version than HTTP/2.3, because 22 is a larger number than 3.
Headers
The previous section focused on the first line of request and response messages
(methods, status codes, reason phrases, and version numbers). Following the start
line comes a list of zero, one, or many HTTP header fields (see Figure 3-5).
HTTP header fields add additional information to request and response messages.
They are basically just lists of name/value pairs. For example, the following header
line assigns the value 19 to the Content-Length header field:
Content-length: 19
Header classifications
The HTTP specification defines several header fields. Applications also are free to
invent their own home-brewed headers. HTTP headers are classified into:
General headers
Can appear in both request and response messages
Request headers
Provide more information about the request
Response headers
Provide more information about the response
Entity headers
Describe body size and contents, or the resource itself
Extension headers
New headers that are not defined in the specification
Each HTTP header has a simple syntax: a name, followed by a colon (:), followed by
optional whitespace, followed by the field value, followed by a CRLF. Table 3-4 lists
some common header examples.
Header continuation lines
Long header lines can be made more readable by breaking them into multiple lines,
preceding each extra line with at least one space or tab character.
Table 3-4. Common header examples
Header example Description
Date: Tue, 3 Oct 1997 02:16:03 GMT The date the server generated the response
Content-length: 15040 The entity body contains 15,040 bytes of data
Content-type: image/gif The entity body is a GIF image
Accept: image/gif, image/jpeg, text/html The client accepts GIF and JPEG images and HTML
52 |Chapter 3: HTTP Messages
For example:
HTTP/1.0 200 OK
Content-Type: image/gif
Content-Length: 8572
Server: Test Server
Version 1.0
In this example, the response message contains a Server header whose value is bro-
ken into continuation lines. The complete value of the header is “Test Server Ver-
sion 1.0”.
We’ll briefly describe all the HTTP headers later in this chapter. We also provide a
more detailed reference summary of all the headers in Appendix C.
Entity Bodies
The third part of an HTTP message is the optional entity body. Entity bodies are the
payload of HTTP messages. They are the things that HTTP was designed to transport.
HTTP messages can carry many kinds of digital data: images, video, HTML docu-
ments, software applications, credit card transactions, electronic mail, and so on.
Version 0.9 Messages
HTTP Version 0.9 was an early version of the HTTP protocol. It was the starting
point for the request and response messages that HTTP has today, but with a far
simpler protocol (see Figure 3-6).
HTTP/0.9 messages also consisted of requests and responses, but the request con-
tained merely the method and the request URL, and the response contained only the
entity. No version information (it was the first and only version at the time), no sta-
tus code or reason phrase, and no headers were included.
Figure 3-6. HTTP/0.9 transaction
GET /specials/saw-blade.gif
Client www.joes-hardware.com
Client www.joes-hardware.com
No version number
Methods |53
However, this simplicity did not allow for much flexibility or the implementation of
most of the HTTP features and applications described in this book. We briefly
describe it here because there are still clients, servers, and other applications that use
it, and application writers should be aware of its limitations.
Methods
Let’s talk in more detail about some of the basic HTTP methods, listed earlier in
Table 3-1. Note that not all methods are implemented by every server. To be compli-
ant with HTTP Version 1.1, a server need implement only the GET and HEAD meth-
ods for its resources.
Even when servers do implement all of these methods, the methods most likely have
restricted uses. For example, servers that support DELETE or PUT (described later in
this section) would not want just anyone to be able to delete or store resources.
These restrictions generally are set up in the server’s configuration, so they vary from
site to site and from server to server.
Safe Methods
HTTP defines a set of methods that are called safe methods. The GET and HEAD
methods are said to be safe, meaning that no action should occur as a result of an
HTTP request that uses either the GET or HEAD method.
By no action, we mean that nothing will happen on the server as a result of the
HTTP request. For example, consider when you are shopping online at Joe’s Hard-
ware and you click on the “submit purchase” button. Clicking on the button sub-
mits a POST request (discussed later) with your credit card information, and an
action is performed on the server on your behalf. In this case, the action is your
credit card being charged for your purchase.
There is no guarantee that a safe method won’t cause an action to be performed (in
practice, that is up to the web developers). Safe methods are meant to allow HTTP
application developers to let users know when an unsafe method that may cause
some action to be performed is being used. In our Joe’s Hardware example, your
web browser may pop up a warning message letting you know that you are making a
request with an unsafe method and that, as a result, something might happen on the
server (e.g., your credit card being charged).
GET
GET is the most common method. It usually is used to ask a server to send a
resource. HTTP/1.1 requires servers to implement this method. Figure 3-7 shows an
example of a client making an HTTP request with the GET method.
54 |Chapter 3: HTTP Messages
HEAD
The HEAD method behaves exactly like the GET method, but the server returns only
the headers in the response. No entity body is ever returned. This allows a client to
inspect the headers for a resource without having to actually get the resource. Using
HEAD, you can:
• Find out about a resource (e.g., determine its type) without getting it.
• See if an object exists, by looking at the status code of the response.
• Test if the resource has been modified, by looking at the headers.
Server developers must ensure that the headers returned are exactly those that a GET
request would return. The HEAD method also is required for HTTP/1.1 compli-
ance. Figure 3-8 shows the HEAD method in action.
PUT
The PUT method writes documents to a server, in the inverse of the way that GET
reads documents from a server. Some publishing systems let you create web pages
and install them directly on a web server using PUT (see Figure 3-9).
Figure 3-7. GET example
Figure 3-8. HEAD example
Client www.joes-hardware.com
HTTP/1.1 200 OK
Content-Type: text/html
Context-Length: 617
<HTML>
<HEAD><TITLE>Joe’s Special Offers </TITLE>
...
GET /seasonal/index-fall.html HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Request message
Response message
Client www.joes-hardware.com
HTTP/1.1 200 OK
Content-Type: text/html
Context-Length: 617
HEAD /seasonal/index-fall.html HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Request message
Response message
no entity body
Methods |55
The semantics of the PUT method are for the server to take the body of the request
and either use it to create a new document named by the requested URL or, if that
URL already exists, use the body to replace it.
Because PUT allows you to change content, many web servers require you to log in
with a password before you can perform a PUT. You can read more about password
authentication in Chapter 12.
POST
The POST method was designed to send input data to the server.*In practice, it is
often used to support HTML forms. The data from a filled-in form typically is sent to
the server, which then marshals it off to where it needs to go (e.g., to a server gateway
program, which then processes it). Figure 3-10 shows a client making an HTTP
request—sending form data to a server—with the POST method.
TRACE
When a client makes a request, that request may have to travel through firewalls,
proxies, gateways, or other applications. Each of these has the opportunity to mod-
ify the original HTTP request. The TRACE method allows clients to see how its
request looks when it finally makes it to the server.
A TRACE request initiates a “loopback” diagnostic at the destination server. The
server at the final leg of the trip bounces back a TRACE response, with the virgin
Figure 3-9. PUT example
* POST is used to send data to a server. PUT is used to deposit data into a resource on the server (e.g., a file).
Joe www.joes-hardware.com
HTTP/1.1 201 Created
Location: http://www.joes-hardware.com/product-list.txt
Content-Type: text/plain
Context-Length: 47
http://www.joes-hardware.com/product-list.txt
PUT /product-list.txt HTTP/1.1
Host: www.joes-hardware.com
Content-type: text/plain
Content-length: 34
Updated product list coming soon!
Request message
Response message Server updates/creates
resource “/product-list.txt”
and writes it to its disk.
56 |Chapter 3: HTTP Messages
request message it received in the body of its response. A client can then see how, or
if, its original message was munged or modified along the request/response chain of
any intervening HTTP applications (see Figure 3-11).
The TRACE method is used primarily for diagnostics; i.e., verifying that requests are
going through the request/response chain as intended. It’s also a good tool for see-
ing the effects of proxies and other applications on your requests.
As good as TRACE is for diagnostics, it does have the drawback of assuming that
intervening applications will treat different types of requests (different methods—
GET, HEAD, POST, etc.) the same. Many HTTP applications do different things
depending on the method—for example, a proxy might pass a POST request directly
to the server but attempt to send a GET request to another HTTP application (such
as a web cache). TRACE does not provide a mechanism to distinguish methods.
Generally, intervening applications make the call as to how they process a TRACE
request.
Figure 3-10. POST example
Client www.joes-hardware.com
HTTP/1.1 20o OK
Content-type: text/plain
Context-length: 37
The bandsaw model 2647 is in stock!
POST /inventory-check.cgi HTTP/1.1
Host: www.joes-hardware.com
Content-type: text/plain
Content-length: 18
item=bandsaw 2647
Request message
Response message
“item= bandsaw 2647”
CGI program
YES!
Inventory
list
Inventory check
Browser sticks data in entity
body of message
Methods |57
No entity body can be sent with a TRACE request. The entity body of the TRACE
response contains, verbatim, the request that the responding server received.
OPTIONS
The OPTIONS method asks the server to tell us about the various supported capabil-
ities of the web server. You can ask a server about what methods it supports in gen-
eral or for particular resources. (Some servers may support particular operations only
on particular kinds of objects).
This provides a means for client applications to determine how best to access vari-
ous resources without actually having to access them. Figure 3-12 shows a request
scenario using the OPTIONS method.
Figure 3-11. TRACE example
Figure 3-12. OPTIONS example
Proxy
TRACE /product-list.txt HTTP/1.1
Accept: *
Host: www.joes-hardware.com
Client
TRACE /product-list.txt HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Via: 1.1 proxy3.company.com
Request message
www.joes-hardware.com
HTTP/1.1 200 OK
Content-type: text/plain
Content-length: 96
Via: 1.1 proxy3.company.com
TRACE /product-list.txt HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Via: 1.1 proxy3.company.com
Response message
HTTP/1.1 200 OK
Content-type: text/plain
Content-length: 96
TRACE /product-list.txt HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Via: 1.1 proxy3.company.com
Examining the entity, the client can see that its request was upgraded to protocol Version 1.1.
Along with the upgrade came a few additional request headers.
Client www.joes-hardware.com
HTTP/1.1 200 OK
Allow: GET, POST, PUT, OPTIONS
Context-length: 0
OPTIONS * HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Request message
Response message
Since the request is for options
on all resources, the server just
returns the methods it supports
for its resources.
58 |Chapter 3: HTTP Messages
DELETE
The DELETE method does just what you would think—it asks the server to delete
the resources specified by the request URL. However, the client application is not
guaranteed that the delete is carried out. This is because the HTTP specification
allows the server to override the request without telling the client. Figure 3-13 shows
an example of the DELETE method.
Extension Methods
HTTP was designed to be field-extensible, so new features wouldn’t cause older soft-
ware to fail. Extension methods are methods that are not defined in the HTTP/1.1
specification. They provide developers with a means of extending the capabilities of
the HTTP services their servers implement on the resources that the servers manage.
Some common examples of extension methods are listed in Table 3-5. These meth-
ods are all part of the WebDAV HTTP extension (see Chapter 19) that helps sup-
port publishing of web content to web servers over HTTP.
It’s important to note that not all extension methods are defined in a formal specifi-
cation. If you define an extension method, it’s likely not to be understood by most
HTTP applications. Likewise, it’s possible that your HTTP applications could run
into extension methods being used by other applications that it does not understand.
Figure 3-13. DELETE example
Table 3-5. Example web publishing extension methods
Method Description
LOCK Allows a user to “lock”a resource—for example, you could lock a resource while you are editing it to prevent
others from editing it at the same time
MKCOL Allows a user to create a resource
COPY Facilitates copying resources on a server
MOVE Moves a resource on a server
Client www.joes-hardware.com
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: 54
I have your delete request,
will take time to process.
DELETE /product-list.txt HTTP/1.1
Host: www.joes-hardware.com
Request message
Response message
Client thinks
resource
was deleted
File “product-list.txt”
removed from
server’s disk
Status Codes |59
In these cases, it is best to be tolerant of extension methods. Proxies should try to
relay messages with unknown methods through to downstream servers if they are
capable of doing that without breaking end-to-end behavior. Otherwise, they should
respond with a 501 Not Implemented status code. Dealing with extension methods
(and HTTP extensions in general) is best done with the old rule, “be conservative in
what you send, be liberal in what you accept.”
Status Codes
HTTP status codes are classified into five broad categories, as shown earlier in
Table 3-2. This section summarizes the HTTP status codes for each of the five classes.
The status codes provide an easy way for clients to understand the results of their
transactions. In this section, we also list example reason phrases, though there is no
real guidance on the exact text for reason phrases. We include the recommended rea-
son phrases from the HTTP/1.1 specification.
100–199: Informational Status Codes
HTTP/1.1 introduced the informational status codes to the protocol. They are rela-
tively new and subject to a bit of controversy about their complexity and perceived
value. Table 3-6 lists the defined informational status codes.
The 100 Continue status code, in particular, is a bit confusing. It’s intended to opti-
mize the case where an HTTP client application has an entity body to send to a
server but wants to check that the server will accept the entity before it sends it. We
discuss it here in a bit more detail (how it interacts with clients, servers, and proxies)
because it tends to confuse HTTP programmers.
Clients and 100 Continue
If a client is sending an entity to a server and is willing to wait for a 100 Continue
response before it sends the entity, the client needs to send an Expect request header
(see Appendix C) with the value 100-continue. If the client is not sending an entity, it
shouldn’t send a 100-continue Expect header, because this will only confuse the
server into thinking that the client might be sending an entity.
Table 3-6. Informational status codes and reason phrases
Status code Reason phrase Meaning
100 Continue Indicates that an initial part of the request was received and the client should con-
tinue. After sending this, the server must respond after receiving the request. See
the Expect header in Appendix C for more information.
101 Switching Protocols Indicates that the server is changing protocols, as specified by the client, to one
listed in the Upgrade header.
60 |Chapter 3: HTTP Messages
100-continue, in many ways, is an optimization. A client application should really
use 100-continue only to avoid sending a server a large entity that the server will not
be able to handle or use.
Because of the initial confusion around the 100 Continue status (and given some of
the older implementations out there), clients that send an Expect header for 100-
continue should not wait forever for the server to send a 100 Continue response.
After some timeout, the client should just send the entity.
In practice, client implementors also should be prepared to deal with unexpected 100
Continue responses (annoying, but true). Some errant HTTP applications send this
code inappropriately.
Servers and 100 Continue
If a server receives a request with the Expect header and 100-continue value, it should
respond with either the 100 Continue response or an error code (see Table 3-9). Serv-
ers should never send a 100 Continue status code to clients that do not send the 100-
continue expectation. However, as we noted above, some errant servers do this.
If for some reason the server receives some (or all) of the entity before it has had a
chance to send a 100 Continue response, it does not need to send this status code,
because the client already has decided to continue. When the server is done reading
the request, however, it still needs to send a final status code for the request (it can
just skip the 100 Continue status).
Finally, if a server receives a request with a 100-continue expectation and it decides to
end the request before it has read the entity body (e.g., because an error has occurred),
it should not just send a response and close the connection, as this can prevent the cli-
ent from receiving the response (see “TCP close and reset errors” in Chapter 4).
Proxies and 100 Continue
A proxy that receives from a client a request that contains the 100-continue expecta-
tion needs to do a few things. If the proxy either knows that the next-hop server (dis-
cussed in Chapter 6) is HTTP/1.1-compliant or does not know what version the
next-hop server is compliant with, it should forward the request with the Expect
header in it. If it knows that the next-hop server is compliant with a version of HTTP
earlier than 1.1, it should respond with the 417 Expectation Failed error.
If a proxy decides to include an Expect header and 100-continue value in its request
on behalf of a client that is compliant with HTTP/1.0 or earlier, it should not for-
ward the 100 Continue response (if it receives one from the server) to the client,
because the client won’t know what to make of it.
It can pay for proxies to maintain some state about next-hop servers and the ver-
sions of HTTP they support (at least for servers that have received recent requests),
so they can better handle requests received with a 100-continue expectation.
Status Codes |61
200–299: Success Status Codes
When clients make requests, the requests usually are successful. Servers have an
array of status codes to indicate success, matched up with different types of requests.
Table 3-7 lists the defined success status codes.
300–399: Redirection Status Codes
The redirection status codes either tell clients to use alternate locations for the
resources they’re interested in or provide an alternate response instead of the con-
tent. If a resource has moved, a redirection status code and an optional Location
header can be sent to tell the client that the resource has moved and where it can
Table 3-7. Success status codes and reason phrases
Status code Reason phrase Meaning
200 OK Request is okay, entity body contains requested resource.
201 Created For requests that create server objects (e.g., PUT). The entity body of the response
should contain the various URLs for referencing the created resource, with the Loca-
tion header containing the most specific reference. See Table 3-21 for more on the
Location header.
The server must have created the object prior to sending this status code.
202 Accepted The request was accepted, but the server has not yet performed any action with it.
There are no guarantees that the server will complete the request; this just means
that the request looked valid when accepted.
The server should include an entity body with a description indicating the status of
the request and possibly an estimate for when it will be completed (or a pointer to
where this information can be obtained).
203 Non-Authoritative
Information
The information contained in the entity headers (see “Entity Headers” for more infor-
mation on entity headers) came not from the origin server but from a copy of the
resource. This could happen if an intermediary had a copy of a resource but could not
or did not validate the meta-information (headers) it sent about the resource.
This response code is not required to be used; it is an option for applications that have
a response that would be a 200 status if the entity headers had come from the origin
server.
204 No Content The response message contains headers and a status line, but no entity body. Prima-
rily used to update browsers without having them move to a new document (e.g.,
refreshing a form page).
205 Reset Content Another code primarily for browsers. Tells the browser to clear any HTML form ele-
ments on the current page.
206 Partial Content A partial or range request was successful. Later, we will see that clients can request
part or a range of a document by using special headers—this status code indicates
that the range request was successful. See “Range Requests” in Chapter 15 for more
on the Range header.
A 206 response must include a Content-Range, Date, and either ETag or Content-
Location header.
62 |Chapter 3: HTTP Messages
now be found (see Figure 3-14). This allows browsers to go to the new location
transparently, without bothering their human users.
Some of the redirection status codes can be used to validate an application’s local
copy of a resource with the origin server. For example, an HTTP application can
check if the local copy of its resource is still up-to-date or if the resource has been
modified on the origin server. Figure 3-15 shows an example of this. The client sends
a special If-Modified-Since header saying to get the document only if it has been
modified since October 1997. The document has not changed since this date, so the
server replies with a 304 status code instead of the contents.
Figure 3-14. Redirected request to new location
Client www.joes-hardware.com
HTTP/1.1 301 OK
Location: http://www.gentle-grooming.com/
Content-length: 56
Content-type: text/plain
Please go to our partner site,
www.gentle-grooming.com
GET /pet-products.txt HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Request message
Response message
Client www.gentle-grooming.com
HTTP/1.1 200 OK
Content-type: text/html
Content-length: 3307
...
GET / HTTP/1.1
Host: www.gentle-grooming.com
Accept: *
Request message
Response message
Status Codes |63
In general, it’s good practice for responses to non-HEAD requests that include a redi-
rection status code to include an entity with a description and links to the redirected
URL(s)—see the first response message in Figure 3-14. Table 3-8 lists the defined
redirection status codes.
Figure 3-15. Request redirected to use local copy
Table 3-8. Redirection status codes and reason phrases
Status code Reason phrase Meaning
300 Multiple Choices Returned when a client has requested a URL that actually refers to multiple
resources, such as a server hosting an English and French version of an HTML docu-
ment. This code is returned along with a list of options; the user can then select
which one he wants. See Chapter 17 for more on clients negotiating when there are
multiple versions. The server can include the preferred URL in the Location header.
301 Moved Permanently Used when the requested URL has been moved. The response should contain in the
Location header the URL where the resource now resides.
302 Found Like the 301 status code; however, the client should use the URL given in the Loca-
tion header to locate the resource temporarily. Future requests should use the old
URL.
Client
www.joes-hardware.com
HTTP/1.1 304 Not Modified
...
Client
GET /seasonal/index-fall.html HTTP/1.1
Host: www.joes-hardware.com
Accept: *
If-Modified-Since: Fri, Oct 3 1997 02:16:00 GMT
Request message
Response message
Client has previously requested copy of:
http://www.joes-hardware.com/seasonal/index-fall.html
Has not changed
Browser displays local copy, since the original
has not changed since we last requested it.
64 |Chapter 3: HTTP Messages
From Table 3-8, you may have noticed a bit of overlap between the 302, 303, and
307 status codes. There is some nuance to how these status codes are used, most of
which stems from differences in the ways that HTTP/1.0 and HTTP/1.1 applications
treat these status codes.
When an HTTP/1.0 client makes a POST request and receives a 302 redirect status
code in response, it will follow the redirect URL in the Location header with a GET
request to that URL (instead of making a POST request, as it did in the original
request).
HTTP/1.0 servers expect HTTP/1.0 clients to do this—when an HTTP/1.0 server
sends a 302 status code after receiving a POST request from an HTTP/1.0 client, the
server expects that client to follow the redirect with a GET request to the redirected
URL.
The confusion comes in with HTTP/1.1. The HTTP/1.1 specification uses the 303
status code to get this same behavior (servers send the 303 status code to redirect a
client’s POST request to be followed with a GET request).
To get around the confusion, the HTTP/1.1 specification says to use the 307 status
code in place of the 302 status code for temporary redirects to HTTP/1.1 clients.
Servers can then save the 302 status code for use with HTTP/1.0 clients.
What this all boils down to is that servers need to check a client’s HTTP version to
properly select which redirect status code to send in a redirect response.
303 See Other Used to tell the client that the resource should be fetched using a different URL. This
new URL is in the Location header of the response message. Its main purpose is to
allow responses to POST requests to direct a client to a resource.
304 Not Modified Clients can make theirrequests conditional by therequest headers they include. See
Table 3-15 for more on conditional headers. If a client makes a conditional request,
such as a GET if the resource has not been changed recently, this code is used to indi-
cate that the resource has not changed. Responses with this status code should not
contain an entity body.
305 Use Proxy Used to indicate that the resource must be accessed through a proxy; the location of
the proxy is given in the Location header. It’s important that clients interpret this
response relative to a specific resource and do not assume that this proxy should be
used for all requests or even all requests to the server holding the requested
resource. This could lead to broken behavior if the proxy mistakenly interfered with a
request, and it poses a security hole.
306 (Unused) Not currently used.
307 Temporary Redirect Like the 301 status code; however, the client should use the URL given in the Loca-
tion header to locate the resource temporarily. Future requests should use the old
URL.
Table 3-8. Redirection status codes and reason phrases (continued)
Status code Reason phrase Meaning
Status Codes |65
400–499: Client Error Status Codes
Sometimes a client sends something that a server just can’t handle, such as a badly
formed request message or, most often, a request for a URL that does not exist.
We’ve all seen the infamous 404 Not Found error code while browsing—this is just
the server telling us that we have requested a resource about which it knows nothing.
Many of the client errors are dealt with by your browser, without it ever bothering
you. A few, like 404, might still pass through. Table 3-9 shows the various client
error status codes.
Table 3-9. Client error status codes and reason phrases
Status code Reason phrase Meaning
400 Bad Request Used to tell the client that it has sent a malformed request.
401 Unauthorized Returned along with appropriate headers that ask the client to authenticate
itself before it can gain access to the resource. See Chapter 12 for more on
authentication.
402 Payment Required Currently this status code is not used, but it has been set aside for future use.
403 Forbidden Used to indicate that the request was refused by the server. If the server wants
to indicate why the request was denied, it can include an entity body describing
the reason. However, this code usually is used when the server does not want to
reveal the reason for the refusal.
404 Not Found Used to indicate that the server cannot find the requested URL. Often, an entity
is included for the client application to display to the user.
405 Method Not Allowed Used when a request is made with a method that is not supported for the
requested URL. The Allow header should be included in the response to tell the
client what methods are allowed on the requested resource. See “Entity Head-
ers” for more on the Allow header.
406 Not Acceptable Clients can specify parameters about what types of entities they are willing to
accept. This code is used when the server has no resource matching the URL that
is acceptable for the client. Often, servers include headers that allow the client
to figure out why the request could not be satisfied. See “Content Negotiation
and Transcoding” in Chapter 17 for more information.
407 Proxy Authentication
Required
Like the 401 status code, but used for proxy servers that require authentication
for a resource.
408 Request Timeout If a client takes too long to complete its request, a server can send back this sta-
tus code and close down the connection. The length of this timeout varies from
server to server but generally is long enough to accommodate any legitimate
request.
409 Conflict Used to indicate some conflict that the request may be causing on a resource.
Servers might send this code when they fear that a request could cause a con-
flict. The response should contain a body describing the conflict.
410 Gone Similar to 404, except that the server once held the resource. Used mostly for
web site maintenance, so a server’s administrator can notify clients when a
resource has been removed.
66 |Chapter 3: HTTP Messages
500–599: Server Error Status Codes
Sometimes a client sends a valid request, but the server itself has an error. This could
be a client running into a limitation of the server or an error in one of the server’s
subcomponents, such as a gateway resource.
Proxies often run into problems when trying to talk to servers on a client’s behalf.
Proxies issue 5XX server error status codes to describe the problem (Chapter 6 cov-
ers this in detail). Table 3-10 lists the defined server error status codes.
411 Length Required Used when the server requires a Content-Length header in the request mes-
sage. See “Content headers” for more on the Content-Length header.
412 Precondition Failed Used if a client makes a conditional request and one of the conditions fails. Con-
ditional requests occur when a client includes an Expect header. See Appendix C
for more on the Expect header.
413 Request Entity Too Large Used when a client sends an entity body that is larger than the server can or
wants to process.
414 Request URI Too Long Used when a client sends a request with a request URL that is larger than the
server can or wants to process.
415 Unsupported Media Type Used when a client sends an entity of a content type that the server does not
understand or support.
416 Requested Range Not
Satisfiable
Used when the request message requested a range of a given resource and that
range either was invalid or could not be met.
417 Expectation Failed Used when the request contained an expectation in the Expect request header
that the server could not satisfy. See Appendix C for more on the Expect header.
A proxy or other intermediary application can send this response code if it has
unambiguous evidence that the origin server will generate a failed expectation
for the request.
Table 3-10. Server error status codes and reason phrases
Status code Reason phrase Meaning
500 Internal Server Error Used when the server encounters an error that prevents it from servicing the
request.
501 Not Implemented Used when aclient makes a request that is beyond the server’s capabilities (e.g.,
using a request method that the server does not support).
502 Bad Gateway Used when a server acting as a proxy or gateway encounters a bogus response
from the next link in the request response chain (e.g., if it is unable to connect to
its parent gateway).
503 Service Unavailable Used to indicate that the server currently cannot service the request but will be
able to in the future. If the server knows when the resource will become avail-
able, it can include a Retry-After header in the response. See “Response Head-
ers” for more on the Retry-After header.
Table 3-9. Client error status codes and reason phrases (continued)
Status code Reason phrase Meaning
Headers |67
Headers
Headers and methods work together to determine what clients and servers do. This
section quickly sketches the purposes of the standard HTTP headers and some head-
ers that are not explicitly defined in the HTTP/1.1 specification (RFC 2616).
Appendix C summarizes all these headers in more detail.
There are headers that are specific for each type of message and headers that are
more general in purpose, providing information in both request and response mes-
sages. Headers fall into five main classes:
General headers
These are generic headers used by both clients and servers. They serve general
purposes that are useful for clients, servers, and other applications to supply to
one another. For example, the Date header is a general-purpose header that allows
both sides to indicate the time and date at which the message was constructed:
Date: Tue, 3 Oct 1974 02:16:00 GMT
Request headers
As the name implies, request headers are specific to request messages. They pro-
vide extra information to servers, such as what type of data the client is willing
to receive. For example, the following Accept header tells the server that the cli-
ent will accept any media type that matches its request:
Accept: */*
Response headers
Response messages have their own set of headers that provide information to the
client (e.g., what type of server the client is talking to). For example, the follow-
ing Server header tells the client that it is talking to a Version 1.0 Tiki-Hut server:
Server: Tiki-Hut/1.0
Entity headers
Entity headers refer to headers that deal with the entity body. For instance,
entity headers can tell the type of the data in the entity body. For example, the
following Content-Type header lets the application know that the data is an
HTML document in the iso-latin-1 character set:
Content-Type: text/html; charset=iso-latin-1
504 Gateway Timeout Similar to status code 408, except that the response is coming from a gateway
or proxy that has timed out waiting for a response to its request from another
server.
505 HTTP Version Not
Supported
Used when a server receives a request in a version of the protocol that it can’tor
won’t support. Some server applications elect not to support older versions of
the protocol.
Table 3-10. Server error status codes and reason phrases (continued)
Status code Reason phrase Meaning
68 |Chapter 3: HTTP Messages
Extension headers
Extension headers are nonstandard headers that have been created by applica-
tion developers but not yet added to the sanctioned HTTP specification. HTTP
programs need to tolerate and forward extension headers, even if they don’t
know what the headers mean.
General Headers
Some headers provide very basic information about a message. These headers are
called general headers. They are the fence straddlers, supplying useful information
about a message regardless of its type.
For example, whether you are constructing a request message or a response mes-
sage, the date and time the message is created means the same thing, so the header
that provides this kind of information is general to both types of messages.
Table 3-11 lists the general informational headers.
General caching headers
HTTP/1.0 introduced the first headers that allowed HTTP applications to cache
local copies of objects instead of always fetching them directly from the origin server.
The latest version of HTTP has a very rich set of cache parameters. In Chapter 7, we
cover caching in depth. Table 3-12 lists the basic caching headers.
Table 3-11. General informational headers
Header Description
Connection Allows clients and servers to specify options about the request/response connection
Datea
aAppendix C lists the acceptable date formats for the Date header.
Provides a date and time stamp telling when the message was created
MIME-Version Gives the version of MIME that the sender is using
Trailer Lists the set of headers that are in the trailer of a message encoded with the chunked transfer encodingb
bChunked transfer codings are discussed further in “Chunking and persistent connections” in Chapter 15.
Transfer-Encoding Tells the receiver what encoding was performed on the message in order for it to be transported safely
Upgrade Gives a new version or protocol that the sender would like to “upgrade” to using
Via Shows what intermediaries (proxies, gateways) the message has gone through
Table 3-12. General caching headers
Header Description
Cache-Control Used to pass caching directions along with the message
Pragmaa
aPragma technically is a request header. It was never specified for use in responses. Because of its common misuse as a response header,
many clients and proxies will interpret Pragma as a response header, but the precise semantics are not well defined. In any case, Pragma
is deprecated in favor of Cache-Control.
Another way to pass directions along with the message, though not specific to caching
Headers |69
Request Headers
Request headers are headers that make sense only in a request message. They give
information about who or what is sending the request, where the request originated,
or what the preferences and capabilities of the client are. Servers can use the informa-
tion the request headers give them about the client to try to give the client a better
response. Table 3-13 lists the request informational headers.
Accept headers
Accept headers give the client a way to tell servers their preferences and capabilities:
what they want, what they can use, and, most importantly, what they don’t want.
Servers can then use this extra information to make more intelligent decisions about
what to send. Accept headers benefit both sides of the connection. Clients get what
they want, and servers don’t waste their time and bandwidth sending something the
client can’t use. Table 3-14 lists the various accept headers.
Table 3-13. Request informational headers
Header Description
Client-IPa
aClient-IP and the UA-* headers are not defined in RFC 2616 but are implemented by many HTTP client applications.
Provides the IP address of the machine on which the client is running
From Provides the email address of the client’s userb
bAn RFC 822 email address format.
Host Gives the hostname and port of the server to which the request is being sent
Referer Provides the URL of the document that contains the current request URI
UA-Color Provides information about the color capabilities of the client machine’s display
UA-CPUc
cWhile implemented by some clients, the UA-* headers can be considered harmful. Content, specifically HTML, should not be targeted at
specific client configurations.
Gives the type or manufacturer of the client’s CPU
UA-Disp Provides information about the client’s display (screen) capabilities
UA-OS Gives the name and version of operating system running on the client machine
UA-Pixels Provides pixel information about the client machine’s display
User-Agent Tells the server the name of the application making the request
Table 3-14. Accept headers
Header Description
Accept Tells the server what media types are okay to send
Accept-Charset Tells the server what charsets are okay to send
Accept-Encoding Tells the server what encodings are okay to send
Accept-Language Tells the server what languages are okay to send
TEa
aSee “Transfer-Encoding Headers” in Chapter 15 for more on the TE header.
Tells the server what extension transfer codings are okay to use
70 |Chapter 3: HTTP Messages
Conditional request headers
Sometimes, clients want to put some restrictions on a request. For instance, if the cli-
ent already has a copy of a document, it might want to ask a server to send the docu-
ment only if it is different from the copy the client already has. Using conditional
request headers, clients can put such restrictions on requests, requiring the server to
make sure that the conditions are true before satisfying the request. Table 3-15 lists
the various conditional request headers.
Request security headers
HTTP natively supports a simple challenge/response authentication scheme for
requests. It attempts to make transactions slightly more secure by requiring clients to
authenticate themselves before getting access to certain resources. We discuss this
challenge/response scheme in Chapter 14, along with other security schemes that
have been implemented on top of HTTP. Table 3-16 lists the request security headers.
Proxy request headers
As proxies become increasingly common on the Internet, a few headers have been
defined to help them function better. In Chapter 6, we discuss these headers in
detail. Table 3-17 lists the proxy request headers.
Table 3-15. Conditional request headers
Header Description
Expect Allows a client to list server behaviors that it requires for a request
If-Match Gets the document if the entity tag matches the current entity tag for the documenta
aSee Chapter 7 for more on entity tags. The tag is basically an identifier for a version of the resource.
If-Modified-Since Restricts the request unless the resource has been modified since the specified date
If-None-Match Gets the document if the entity tags supplied do not match those of the current document
If-Range Allows a conditional request for a range of a document
If-Unmodified-Since Restricts the request unless the resource has not been modified since the specified date
Range Requests a specific range of a resource, if the server supports range requestsb
bSee “Range Requests” in Chapter 15 for more on the Range header.
Table 3-16. Request security headers
Header Description
Authorization Contains the data the client is supplying to the server to authenticate itself
Cookie Used by clients to pass a token to the server—not a true security header, but it does have security
implicationsa
aThe Cookie header is not defined in RFC 2616; it is discussed in detail in Chapter 11.
Cookie2 Used to note the version of cookies a requestor supports; see “Version 1 (RFC 2965) Cookies” in
Chapter 11
Headers |71
Response Headers
Response messages have their own set of response headers. Response headers pro-
vide clients with extra information, such as who is sending the response, the capabil-
ities of the responder, or even special instructions regarding the response. These
headers help the client deal with the response and make better requests in the future.
Table 3-18 lists the response informational headers.
Negotiation headers
HTTP/1.1 provides servers and clients with the ability to negotiate for a resource if
multiple representations are available—for instance, when there are both French and
German translations of an HTML document on a server. Chapter 17 walks through
negotiation in detail. Here are a few headers servers use to convey information about
resources that are negotiable. Table 3-19 lists the negotiation headers.
Table 3-17. Proxy request headers
Header Description
Max-Forwards The maximum number of times a request should be forwarded to another proxy or gateway on its way
to the origin server—used with the TRACE methoda
aSee “Max-Forwards” in Chapter 6.
Proxy-Authorization Same as Authorization, but used when authenticating with a proxy
Proxy-Connection Same as Connection, but used when establishing connections with a proxy
Table 3-18. Response informational headers
Header Description
Age How old the response isa
aImplies that the response has traveled through an intermediary, possibly from a proxy cache.
Publicb
bThe Public header is defined in RFC 2068 but does not appear in the latest HTTP definition (RFC 2616).
A list of request methods the server supports for its resources
Retry-After A date or time to try back, if a resource is unavailable
Server The name and version of the server’s application software
Titlec
cThe Title header is not defined in RFC 2616; see the original HTTP/1.0 draft definition (http://www.w3.org/Protocols/HTTP/HTTP2.html).
For HTML documents, the title as given by the HTML document source
Warning A more detailed warning message than what is in the reason phrase
Table 3-19. Negotiation headers
Header Description
Accept-Ranges The type of ranges that a server will accept for this resource
Vary A list of other headers that the server looks at and that may cause the response to vary; i.e., a list of
headers the server looks at to pick which is the best version of a resource to send the client
72 |Chapter 3: HTTP Messages
Response security headers
You’ve already seen the request security headers, which are basically the response
side of HTTP’s challenge/response authentication scheme. We talk about security in
detail in Chapter 14. For now, here are the basic challenge headers. Table 3-20 lists
the response security headers.
Entity Headers
There are many headers to describe the payload of HTTP messages. Because both
request and response messages can contain entities, these headers can appear in
either type of message.
Entity headers provide a broad range of information about the entity and its content,
from information about the type of the object to valid request methods that can be
made on the resource. In general, entity headers tell the receiver of the message what
it’s dealing with. Table 3-21 lists the entity informational headers.
Content headers
The content headers provide specific information about the content of the entity,
revealing its type, size, and other information useful for processing it. For instance, a
web browser can look at the content type returned and know how to display the
object. Table 3-22 lists the various content headers.
Table 3-20. Response security headers
Header Description
Proxy-Authenticate A list of challenges for the client from the proxy
Set-Cookie Not a true security header, but it has security implications; used to set a token on the client side that
the server can use to identify the clienta
aSet-Cookie and Set-Cookie2 are extension headers that are also covered in Chapter 11.
Set-Cookie2 Similar to Set-Cookie, RFC 2965 Cookie definition; see “Version 1 (RFC 2965) Cookies” in Chapter 11
WWW-Authenticate A list of challenges for the client from the server
Table 3-21. Entity informational headers
Header Description
Allow Lists the request methods that can be performed on this entity
Location Tells the client where the entity really is located; used in directing the receiver to a (possibly new)
location (URL) for the resource
Table 3-22. Content headers
Header Description
Content-BaseaThe base URL for resolving relative URLs within the body
Content-Encoding Any encoding that was performed on the body
For More Information |73
Entity caching headers
The general caching headers provide directives about how or when to cache. The
entity caching headers provide information about the entity being cached—for
example, information needed to validate whether a cached copy of the resource is
still valid and hints about how better to estimate when a cached resource may no
longer be valid.
In Chapter 7, we dive deep into the heart of caching HTTP requests and responses.
We will see these headers again there. Table 3-23 lists the entity caching headers.
.
For More Information
For more information, refer to:
http://www.w3.org/Protocols/rfc2616/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol,” by R. Fielding, J. Gettys, J. Mogul, H.
Frystyk, L. Mastinter, P. Leach, and T. Berners-Lee.
HTTP Pocket Reference
Clintin Wong, O’Reilly & Associates, Inc.
http://www.w3.org/Protocols/
The W3C architecture page for HTTP.
Content-Language The natural language that is best used to understand the body
Content-Length The length or size of the body
Content-Location Where the resource actually is located
Content-MD5 An MD5 checksum of the body
Content-Range The range of bytes that this entity represents from the entire resource
Content-Type The type of object that this body is
aThe Content-Base header is not defined in RFC 2616.
Table 3-23. Entity caching headers
Header Description
ETag The entity tag associated with this entitya
aEntity tags are basically identifiers for a particular version of a resource.
Expires The date and time at which this entity will no longer be valid and will need to be fetched from the
original source
Last-Modified The last date and time when this entity changed
Table 3-22. Content headers (continued)
Header Description
74
CHAPTER 4
Connection Management
The HTTP specifications explain HTTP messages fairly well, but they don’t talk
much about HTTP connections, the critical plumbing that HTTP messages flow
through. If you’re a programmer writing HTTP applications, you need to under-
stand the ins and outs of HTTP connections and how to use them.
HTTP connection management has been a bit of a black art, learned as much from
experimentation and apprenticeship as from published literature. In this chapter,
you’ll learn about:
• How HTTP uses TCP connections
• Delays, bottlenecks and clogs in TCP connections
• HTTP optimizations, including parallel, keep-alive, and pipelined connections
• Dos and don’ts for managing connections
TCP Connections
Just about all of the world’s HTTP communication is carried over TCP/IP, a popular
layered set of packet-switched network protocols spoken by computers and network
devices around the globe. A client application can open a TCP/IP connection to a
server application, running just about anywhere in the world. Once the connection is
established, messages exchanged between the client’s and server’s computers will
never be lost, damaged, or received out of order.*
Say you want the latest power tools price list from Joe’s Hardware store:
http://www.joes-hardware.com:80/power-tools.html
When given this URL, your browser performs the steps shown in Figure 4-1. In Steps
1–3, the IP address and port number of the server are pulled from the URL. A TCP
* Though messages won’t be lost or corrupted, communication between client and server can be severed if a
computer or network breaks. In this case, the client and server are notified of the communication breakdown.
TCP Connections |75
connection is made to the web server in Step 4, and a request message is sent across
the connection in Step 5. The response is read in Step 6, and the connection is closed
in Step 7.
TCP Reliable Data Pipes
HTTP connections really are nothing more than TCP connections, plus a few rules
about how to use them. TCP connections are the reliable connections of the Inter-
net. To send data accurately and quickly, you need to know the basics of TCP.*
TCP gives HTTP a reliable bit pipe. Bytes stuffed in one side of a TCP connection
come out the other side correctly, and in the right order (see Figure 4-2).
Figure 4-1. Web browsers talk to web servers over TCP connections
* If you are trying to write sophisticated HTTP applications, and especially if you want them to be fast, you’ll
want to learn a lot more about the internals and performance of TCP than we discuss in this chapter. We
recommend the “TCP/IP Illustrated” books by W. Richard Stevens (Addison Wesley).
Client Server
www.joes-hardware.com
Client Server
Client Server
Client Server
Internet
(7) The browser closes the connection
(1) The browser extracts the hostname
(2) The browser looks up the IP address for this hostname (DNS)
(3) The browser gets the port number (80)
(4) The browser makes a TCP connection to 202.43.78.3 port 80
(5) The browser sends an HTTP GET request message to the server
(6) The browser reads the HTTP response message from the server
202.43.78.3
80
http://www.joes-hardware.com:80/power-tools.html
80
(202.43.78.3)
Internet
Internet
Internet
76 |Chapter 4: Connection Management
TCP Streams Are Segmented and Shipped by IP Packets
TCP sends its data in little chunks called IP packets (or IP datagrams). In this way,
HTTP is the top layer in a “protocol stack” of “HTTP over TCP over IP,” as depicted
in Figure 4-3a. A secure variant, HTTPS, inserts a cryptographic encryption layer
(called TLS or SSL) between HTTP and TCP (Figure 4-3b).
When HTTP wants to transmit a message, it streams the contents of the message
data, in order, through an open TCP connection. TCP takes the stream of data,
chops up the data stream into chunks called segments, and transports the segments
across the Internet inside envelopes called IP packets (see Figure 4-4). This is all han-
dled by the TCP/IP software; the HTTP programmer sees none of it.
Each TCP segment is carried by an IP packet from one IP address to another IP
address. Each of these IP packets contains:
• An IP packet header (usually 20 bytes)
• A TCP segment header (usually 20 bytes)
• A chunk of TCP data (0 or more bytes)
The IP header contains the source and destination IP addresses, the size, and other
flags. The TCP segment header contains TCP port numbers, TCP control flags, and
numeric values used for data ordering and integrity checking.
Figure 4-2. TCP carries HTTP data in order, and without corruption
Figure 4-3. HTTP and HTTPS network protocol stacks
Client Server
Internet
...TH lmth.xedni/ TEG
HTTP Application layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(a) HTTP
HTTP Application layer
TSL or SSL Security layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(b) HTTPS
TCP Connections |77
Keeping TCP Connections Straight
A computer might have several TCP connections open at any one time. TCP keeps
all these connections straight through port numbers.
Port numbers are like employees’ phone extensions. Just as a company’s main phone
number gets you to the front desk and the extension gets you to the right employee,
the IP address gets you to the right computer and the port number gets you to the
right application. A TCP connection is distinguished by four values:
<source-IP-address, source-port, destination-IP-address, destination-port>
Together, these four values uniquely define a connection. Two different TCP connec-
tions are not allowed to have the same values for all four address components (but
different connections can have the same values for some of the components).
Figure 4-4. IP packets carry TCP segments, which carry chunks of the TCP data stream
Client Server
Version Hdr length
(words) Type of service
(TOS) Total datagram length
(bytes)
Packet ID
(16-bit number) Flags Fragmentation offset
Time to live
(TTL) Upper-level protocol Header checksum
Source IP address
Destination IP address
Source port Destination port
TCP sequence number
Piggybacked acknowledgment
Window size
Hdr length
(words) Reserved
URG
ACK
PSH
RST
SYN
FIN
TCP checksum Urgent pointer
GET /index.html HTTP/1.1<CR><LF>
Host: www.joes-hardware.c
Chunk of TCP data stream
TCP segment
IP packet
TCP
segment
#1
TCP
segment
#2
TCP
segment
#3
78 |Chapter 4: Connection Management
In Figure 4-5, there are four connections: A, B, C and D. The relevant information
for each port is listed in Table 4-1.
Note that some of the connections share the same destination port number (C and D
both have destination port 80). Some of the connections have the same source IP
address (B and C). Some have the same destination IP address (A and B, and C and
D). But no two different connections share all four identical values.
Programming with TCP Sockets
Operating systems provide different facilities for manipulating their TCP connec-
tions. Let’s take a quick look at one TCP programming interface, to make things
concrete. Table 4-2 shows some of the primary interfaces provided by the sockets
API. This sockets API hides all the details of TCP and IP from the HTTP program-
mer. The sockets API was first developed for the Unix operating system, but variants
are now available for almost every operating system and language.
Table 4-1. TCP connection values
Connection Source IP address Source port Destination IP address Destination port
A 209.1.32.34 2034 204.62.128.58 4133
B 209.1.32.35 3227 204.62.128.58 4140
C 209.1.32.35 3105 207.25.71.25 80
D 209.1.33.89 5100 207.25.71.25 80
Figure 4-5. Four distinct TCP connections
Table 4-2. Common socket interface functions for programming TCP connections
Sockets API call Description
s = socket(<parameters>) Creates a new, unnamed, unattached socket.
bind(s, <local IP:port>) Assigns a local port number and interface to the socket.
209.1.32.34
204.62.128.58
209.1.32.35 209.1.33.89
207.25.71.25
A B C D
2034
4133 4140
3227 3105
80
5100
TCP Connections |79
The sockets API lets you create TCP endpoint data structures, connect these end-
points to remote server TCP endpoints, and read and write data streams. The TCP
API hides all the details of the underlying network protocol handshaking and the seg-
mentation and reassembly of the TCP data stream to and from IP packets.
In Figure 4-1, we showed how a web browser could download the power-tools.html
web page from Joe’s Hardware store using HTTP. The pseudocode in Figure 4-6
sketches how we might use the sockets API to highlight the steps the client and
server could perform to implement this HTTP transaction.
connect(s, <remote IP:port>) Establishes a TCP connection to a local socket and a remote host and port.
listen(s,...) Marks a local socket as legal to accept connections.
s2 = accept(s) Waits for someone to establish a connection to a local port.
n = read(s,buffer,n) Tries to read n bytes from the socket into the buffer.
n = write(s,buffer,n) Tries to write n bytes from the buffer into the socket.
close(s) Completely closes the TCP connection.
shutdown(s,<side>) Closes just the input or the output of the TCP connection.
getsockopt(s, ...) Reads the value of an internal socket configuration option.
setsockopt(s, ...) Changes the value of an internal socket configuration option.
Figure 4-6. How TCP clients and servers communicate using the TCP sockets interface
Table 4-2. Common socket interface functions for programming TCP connections (continued)
Sockets API call Description
Client Server
(C1) get IP address & port
(C2) create new socket (socket)
(C3) connect to server IP:port (connect)
(C4) connection successful
(C5) send HTTP request (write)
(C6) wait for HTTP response (read)
(C7) process HTTP response
(C8) close connection (close)
(S1) create new socket (socket)
(S2) bind socket to port 80 (bind)
(S3) permit socket connections (listen)
(S4) wait for connection (accept)
(S5) application notified of connection
(S6) start reading request (read)
(S7) process HTTP request message
(S8) send back HTTP response (write)
(S9) close connection (close)
80 |Chapter 4: Connection Management
We begin with the web server waiting for a connection (Figure 4-6, S4). The client
determines the IP address and port number from the URL and proceeds to establish
a TCP connection to the server (Figure 4-6, C3). Establishing a connection can take a
while, depending on how far away the server is, the load on the server, and the con-
gestion of the Internet.
Once the connection is set up, the client sends the HTTP request (Figure 4-6, C5)
and the server reads it (Figure 4-6, S6). Once the server gets the entire request mes-
sage, it processes the request, performs the requested action (Figure 4-6, S7), and
writes the data back to the client. The client reads it (Figure 4-6, C6) and processes
the response data (Figure 4-6, C7).
TCP Performance Considerations
Because HTTP is layered directly on TCP, the performance of HTTP transactions
depends critically on the performance of the underlying TCP plumbing. This section
highlights some significant performance considerations of these TCP connections. By
understanding some of the basic performance characteristics of TCP, you’ll better
appreciate HTTP’s connection optimization features, and you’ll be able to design
and implement higher-performance HTTP applications.
This section requires some understanding of the internal details of the TCP proto-
col. If you are not interested in (or are comfortable with) the details of TCP perfor-
mance considerations, feel free to skip ahead to “HTTP Connection Handling.”
Because TCP is a complex topic, we can provide only a brief overview of TCP perfor-
mance here. Refer to the section “For More Information” at the end of this chapter
for a list of excellent TCP references.
HTTP Transaction Delays
Let’s start our TCP performance tour by reviewing what networking delays occur in
the course of an HTTP request. Figure 4-7 depicts the major connect, transfer, and
processing delays for an HTTP transaction.
Figure 4-7. Timeline of a serial HTTP transaction
Client
Server
Connect Request Process Response Close Time
DNS lookup
TCP Performance Considerations |81
Notice that the transaction processing time can be quite small compared to the time
required to set up TCP connections and transfer the request and response messages.
Unless the client or server is overloaded or executing complex dynamic resources,
most HTTP delays are caused by TCP network delays.
There are several possible causes of delay in an HTTP transaction:
1. A client first needs to determine the IP address and port number of the web
server from the URI. If the hostname in the URI was not recently visited, it may
take tens of seconds to convert the hostname from a URI into an IP address
using the DNS resolution infrastructure.*
2. Next, the client sends a TCP connection request to the server and waits for the
server to send back a connection acceptance reply. Connection setup delay
occurs for every new TCP connection. This usually takes at most a second or
two, but it can add up quickly when hundreds of HTTP transactions are made.
3. Once the connection is established, the client sends the HTTP request over the
newly established TCP pipe. The web server reads the request message from the
TCP connection as the data arrives and processes the request. It takes time for
the request message to travel over the Internet and get processed by the server.
4. The web server then writes back the HTTP response, which also takes time.
The magnitude of these TCP network delays depends on hardware speed, the load of
the network and server, the size of the request and response messages, and the dis-
tance between client and server. The delays also are significantly affected by techni-
cal intricacies of the TCP protocol.
Performance Focus Areas
The remainder of this section outlines some of the most common TCP-related delays
affecting HTTP programmers, including the causes and performance impacts of:
• The TCP connection setup handshake
• TCP slow-start congestion control
• Nagle’s algorithm for data aggregation
• TCP’s delayed acknowledgment algorithm for piggybacked acknowledgments
• TIME_WAIT delays and port exhaustion
If you are writing high-performance HTTP software, you should understand each of
these factors. If you don’t need this level of performance optimization, feel free to
skip ahead.
* Luckily, most HTTP clients keep a small DNS cache of IP addresses for recently accessed sites. When the IP
address is already “cached” (recorded) locally, the lookup is instantaneous. Because most web browsing is
to a small number of popular sites, hostnames usually are resolved very quickly.
82 |Chapter 4: Connection Management
TCP Connection Handshake Delays
When you set up a new TCP connection, even before you send any data, the TCP
software exchanges a series of IP packets to negotiate the terms of the connection
(see Figure 4-8). These exchanges can significantly degrade HTTP performance if the
connections are used for small data transfers.
Here are the steps in the TCP connection handshake:
1. To request a new TCP connection, the client sends a small TCP packet (usually
40–60 bytes) to the server. The packet has a special “SYN” flag set, which means
it’s a connection request. This is shown in Figure 4-8a.
2. If the server accepts the connection, it computes some connection parameters
and sends a TCP packet back to the client, with both the “SYN” and “ACK”
flags set, indicating that the connection request is accepted (see Figure 4-8b).
3. Finally, the client sends an acknowledgment back to the server, letting it know
that the connection was established successfully (see Figure 4-8c). Modern TCP
stacks let the client send data in this acknowledgment packet.
The HTTP programmer never sees these packets—they are managed invisibly by the
TCP/IP software. All the HTTP programmer sees is a delay when creating a new TCP
connection.
The SYN/SYN+ACK handshake (Figure 4-8a and b) creates a measurable delay
when HTTP transactions do not exchange much data, as is commonly the case. The
TCP connect ACK packet (Figure 4-8c) often is large enough to carry the entire
HTTP request message,*and many HTTP server response messages fit into a single
IP packet (e.g., when the response is a small HTML file of a decorative graphic, or a
304 Not Modified response to a browser cache request).
Figure 4-8. TCP requires two packet transfers to set up the connection before it can send data
* IP packets are usually a few hundred bytes for Internet traffic and around 1,500 bytes for local traffic.
Client
Server
Connection handshake delay Data transfer Time
Connect
(a) SYN
(b) SYN+ACK
(c) ACK
GET / HTTP. . .
(d) HTTP/1.1 304 Not modified
. . .
TCP Performance Considerations |83
The end result is that small HTTP transactions may spend 50% or more of their time
doing TCP setup. Later sections will discuss how HTTP allows reuse of existing con-
nections to eliminate the impact of this TCP setup delay.
Delayed Acknowledgments
Because the Internet itself does not guarantee reliable packet delivery (Internet rout-
ers are free to destroy packets at will if they are overloaded), TCP implements its
own acknowledgment scheme to guarantee successful data delivery.
Each TCP segment gets a sequence number and a data-integrity checksum. The
receiver of each segment returns small acknowledgment packets back to the sender
when segments have been received intact. If a sender does not receive an acknowl-
edgment within a specified window of time, the sender concludes the packet was
destroyed or corrupted and resends the data.
Because acknowledgments are small, TCP allows them to “piggyback” on outgoing
data packets heading in the same direction. By combining returning acknowledg-
ments with outgoing data packets, TCP can make more efficient use of the network.
To increase the chances that an acknowledgment will find a data packet headed in
the same direction, many TCP stacks implement a “delayed acknowledgment” algo-
rithm. Delayed acknowledgments hold outgoing acknowledgments in a buffer for a
certain window of time (usually 100–200 milliseconds), looking for an outgoing data
packet on which to piggyback. If no outgoing data packet arrives in that time, the
acknowledgment is sent in its own packet.
Unfortunately, the bimodal request-reply behavior of HTTP reduces the chances that
piggybacking can occur. There just aren’t many packets heading in the reverse direc-
tion when you want them. Frequently, the disabled acknowledgment algorithms
introduce significant delays. Depending on your operating system, you may be able
to adjust or disable the delayed acknowledgment algorithm.
Before you modify any parameters of your TCP stack, be sure you know what you
are doing. Algorithms inside TCP were introduced to protect the Internet from
poorly designed applications. If you modify any TCP configurations, be absolutely
sure your application will not create the problems the algorithms were designed to
avoid.
TCP Slow Start
The performance of TCP data transfer also depends on the age of the TCP connec-
tion. TCP connections “tune” themselves over time, initially limiting the maximum
speed of the connection and increasing the speed over time as data is transmitted
successfully. This tuning is called TCP slow start, and it is used to prevent sudden
overloading and congestion of the Internet.
84 |Chapter 4: Connection Management
TCP slow start throttles the number of packets a TCP endpoint can have in flight at
any one time. Put simply, each time a packet is received successfully, the sender gets
permission to send two more packets. If an HTTP transaction has a large amount of
data to send, it cannot send all the packets at once. It must send one packet and wait
for an acknowledgment; then it can send two packets, each of which must be acknowl-
edged, which allows four packets, etc. This is called “opening the congestion window.”
Because of this congestion-control feature, new connections are slower than “tuned”
connections that already have exchanged a modest amount of data. Because tuned
connections are faster, HTTP includes facilities that let you reuse existing connec-
tions. We’ll talk about these HTTP “persistent connections” later in this chapter.
Nagle’s Algorithm and TCP_NODELAY
TCP has a data stream interface that permits applications to stream data of any size
to the TCP stack—even a single byte at a time! But because each TCP segment car-
ries at least 40 bytes of flags and headers, network performance can be degraded
severely if TCP sends large numbers of packets containing small amounts of data.*
Nagle’s algorithm (named for its creator, John Nagle) attempts to bundle up a large
amount of TCP data before sending a packet, aiding network efficiency. The algo-
rithm is described in RFC 896, “Congestion Control in IP/TCP Internetworks.”
Nagle’s algorithm discourages the sending of segments that are not full-size (a
maximum-size packet is around 1,500 bytes on a LAN, or a few hundred bytes
across the Internet). Nagle’s algorithm lets you send a non-full-size packet only if all
other packets have been acknowledged. If other packets are still in flight, the partial
data is buffered. This buffered data is sent only when pending packets are acknowl-
edged or when the buffer has accumulated enough data to send a full packet.†
Nagle’s algorithm causes several HTTP performance problems. First, small HTTP
messages may not fill a packet, so they may be delayed waiting for additional data
that will never arrive. Second, Nagle’s algorithm interacts poorly with disabled
acknowledgments—Nagle’s algorithm will hold up the sending of data until an
acknowledgment arrives, but the acknowledgment itself will be delayed 100–200
milliseconds by the delayed acknowledgment algorithm.‡
HTTP applications often disable Nagle’s algorithm to improve performance, by setting
the TCP_NODELAY parameter on their stacks. If you do this, you must ensure that
you write large chunks of data to TCP so you don’t create a flurry of small packets.
* Sending a storm of single-byte packets is called “sender silly window syndrome.” This is inefficient, anti-
social, and can be disruptive to other Internet traffic.
† Several variations of this algorithm exist, including timeouts and acknowledgment logic changes, but the
basic algorithm causes buffering of data smaller than a TCP segment.
‡ These problems can become worse when using pipelined connections (described later in this chapter),
because clients may have several messages to send to the same server and do not want delays.
TCP Performance Considerations |85
TIME_WAIT Accumulation and Port Exhaustion
TIME_WAIT port exhaustion is a serious performance problem that affects perfor-
mance benchmarking but is relatively uncommon in real deployments. It warrants
special attention because most people involved in performance benchmarking even-
tually run into this problem and get unexpectedly poor performance.
When a TCP endpoint closes a TCP connection, it maintains in memory a small con-
trol block recording the IP addresses and port numbers of the recently closed con-
nection. This information is maintained for a short time, typically around twice the
estimated maximum segment lifetime (called “2MSL”; often two minutes*), to make
sure a new TCP connection with the same addresses and port numbers is not cre-
ated during this time. This prevents any stray duplicate packets from the previous
connection from accidentally being injected into a new connection that has the same
addresses and port numbers. In practice, this algorithm prevents two connections
with the exact same IP addresses and port numbers from being created, closed, and
recreated within two minutes.
Today’s higher-speed routers make it extremely unlikely that a duplicate packet will
show up on a server’s doorstep minutes after a connection closes. Some operating
systems set 2MSL to a smaller value, but be careful about overriding this value. Pack-
ets do get duplicated, and TCP data will be corrupted if a duplicate packet from a
past connection gets inserted into a new stream with the same connection values.
The 2MSL connection close delay normally is not a problem, but in benchmarking
situations, it can be. It’s common that only one or a few test load-generation com-
puters are connecting to a system under benchmark test, which limits the number of
client IP addresses that connect to the server. Furthermore, the server typically is lis-
tening on HTTP’s default TCP port, 80. These circumstances limit the available
combinations of connection values, at a time when port numbers are blocked from
reuse by TIME_WAIT.
In a pathological situation with one client and one web server, of the four values that
make up a TCP connection:
<source-IP-address, source-port, destination-IP-address, destination-port>
three of them are fixed—only the source port is free to change:
<client-IP, source-port, server-IP, 80>
Each time the client connects to the server, it gets a new source port in order to have
a unique connection. But because a limited number of source ports are available
(say, 60,000) and no connection can be reused for 2MSL seconds (say, 120 sec-
onds), this limits the connect rate to 60,000 / 120 = 500 transactions/sec. If you keep
* The 2MSL value of two minutes is historical. Long ago, when routers were much slower, it was estimated
that a duplicate copy of a packet might be able to remain queued in the Internet for up to a minute before
being destroyed. Today, the maximum segment lifetime is much smaller.
86 |Chapter 4: Connection Management
making optimizations, and your server doesn’t get faster than about 500 transac-
tions/sec, make sure you are not experiencing TIME_WAIT port exhaustion. You
can fix this problem by using more client load-generator machines or making sure
the client and server rotate through several virtual IP addresses to add more connec-
tion combinations.
Even if you do not suffer port exhaustion problems, be careful about having large
numbers of open connections or large numbers of control blocks allocated for con-
nection in wait states. Some operating systems slow down dramatically when there
are numerous open connections or control blocks.
HTTP Connection Handling
The first two sections of this chapter provided a fire-hose tour of TCP connections
and their performance implications. If you’d like to learn more about TCP network-
ing, check out the resources listed at the end of the chapter.
We’re going to switch gears now and get squarely back to HTTP. The rest of this
chapter explains the HTTP technology for manipulating and optimizing connec-
tions. We’ll start with the HTTP Connection header, an often misunderstood but
important part of HTTP connection management. Then we’ll talk about HTTP’s
connection optimization techniques.
The Oft-Misunderstood Connection Header
HTTP allows a chain of HTTP intermediaries between the client and the ultimate
origin server (proxies, caches, etc.). HTTP messages are forwarded hop by hop from
the client, through intermediary devices, to the origin server (or the reverse).
In some cases, two adjacent HTTP applications may want to apply a set of options to
their shared connection. The HTTP Connection header field has a comma-separated
list of connection tokens that specify options for the connection that aren’t propa-
gated to other connections. For example, a connection that must be closed after
sending the next message can be indicated by Connection: close.
The Connection header sometimes is confusing, because it can carry three different
types of tokens:
• HTTP header field names, listing headers relevant for only this connection
• Arbitrary token values, describing nonstandard options for this connection
• The value close, indicating the persistent connection will be closed when done
If a connection token contains the name of an HTTP header field, that header field
contains connection-specific information and must not be forwarded. Any header
fields listed in the Connection header must be deleted before the message is for-
warded. Placing a hop-by-hop header name in a Connection header is known as
HTTP Connection Handling |87
“protecting the header,” because the Connection header protects against accidental
forwarding of the local header. An example is shown in Figure 4-9.
When an HTTP application receives a message with a Connection header, the
receiver parses and applies all options requested by the sender. It then deletes the
Connection header and all headers listed in the Connection header before forward-
ing the message to the next hop. In addition, there are a few hop-by-hop headers that
might not be listed as values of a Connection header, but must not be proxied. These
include Proxy-Authenticate, Proxy-Connection, Transfer-Encoding, and Upgrade.
For more about the Connection header, see Appendix C.
Serial Transaction Delays
TCP performance delays can add up if the connections are managed naively. For
example, suppose you have a web page with three embedded images. Your browser
needs to issue four HTTP transactions to display this page: one for the top-level
HTML and three for the embedded images. If each transaction requires a new con-
nection, the connection and slow-start delays can add up (see Figure 4-10).*
Figure 4-9. The Connection header allows the sender to specify connection-specific options
Figure 4-10. Four transactions (serial)
* For the purpose of this example, assume all objects are roughly the same size and are hosted from the same
server, and that the DNS entry is cached, eliminating the DNS lookup time.
Client Server
Proxy
HTTP/1.1 200 OK
Cache-control: max-age=3600
Connection: meter, close, bill-my-credit-card
Meter: max-uses=3, max-refuses=6, dont-report
The Connection header says the Meter header
should not be forwarded, the hypothetical
“bill-my-credit-card” option applies, and the
persistent connection will be closed when this
transaction is done.
Client
Server
Transaction 1
Time
Connect- 1 Connect- 2 Connect- 3 Connect- 4
Transaction 2 Transaction 3 Transaction 4
Request- 1
Request- 2
Request- 3
Request- 4
Response- 1
Response- 2
Response- 3
Response- 4
88 |Chapter 4: Connection Management
In addition to the real delay imposed by serial loading, there is also a psychological
perception of slowness when a single image is loading and nothing is happening on
the rest of the page. Users prefer multiple images to load at the same time.*
Another disadvantage of serial loading is that some browsers are unable to display
anything onscreen until enough objects are loaded, because they don’t know the
sizes of the objects until they are loaded, and they may need the size information to
decide where to position the objects on the screen. In this situation, the browser may
be making good progress loading objects serially, but the user may be faced with a
blank white screen, unaware that any progress is being made at all.†
Several current and emerging techniques are available to improve HTTP connection
performance. The next several sections discuss four such techniques:
Parallel connections
Concurrent HTTP requests across multiple TCP connections
Persistent connections
Reusing TCP connections to eliminate connect/close delays
Pipelined connections
Concurrent HTTP requests across a shared TCP connection
Multiplexed connections
Interleaving chunks of requests and responses (experimental)
Parallel Connections
As we mentioned previously, a browser could naively process each embedded object
serially by completely requesting the original HTML page, then the first embedded
object, then the second embedded object, etc. But this is too slow!
HTTP allows clients to open multiple connections and perform multiple HTTP
transactions in parallel, as sketched in Figure 4-11. In this example, four embedded
images are loaded in parallel, with each transaction getting its own TCP connection.‡
Parallel Connections May Make Pages Load Faster
Composite pages consisting of embedded objects may load faster if they take advan-
tage of the dead time and bandwidth limits of a single connection. The delays can be
* This is true even if loading multiple images at the same time is slower than loading images one at a time!
Users often perceive multiple-image loading as faster.
† HTML designers can help eliminate this “layout delay” by explicitly adding width and height attributes to
HTML tags for embedded objects such as images. Explicitly providing the width and height of the embedded
image allows the browser to make graphical layout decisions before it receives the objects from the server.
‡ The embedded components do not all need to be hosted on the same web server, so the parallel connections
can be established to multiple servers.
Parallel Connections |89
overlapped, and if a single connection does not saturate the client’s Internet band-
width, the unused bandwidth can be allocated to loading additional objects.
Figure 4-12 shows a timeline for parallel connections, which is significantly faster
than Figure 4-10. The enclosing HTML page is loaded first, and then the remaining
three transactions are processed concurrently, each with their own connection.*
Because the images are loaded in parallel, the connection delays are overlapped.
Parallel Connections Are Not Always Faster
Even though parallel connections may be faster, however, they are not always faster.
When the client’s network bandwidth is scarce (for example, a browser connected to
Figure 4-11. Each component of a page involves a separate HTTP transaction
* There will generally still be a small delay between each connection request due to software overheads, but
the connection requests and transfer times are mostly overlapped.
Figure 4-12. Four transactions (parallel)
Client
Server 1
Server 2
Internet
Client
Server
Transaction 1
Time
Connect- 1 Connect- 2
Connect- 3
Connect- 4
Transaction 2, 3, 4
(parallel connections)
Request- 1
Request- 2
Request- 3
Response- 1
Re se- 2
Response- 3
Response- 4
Request- 4
(Usually a small software delay
between each connection)
90 |Chapter 4: Connection Management
the Internet through a 28.8-Kbps modem), most of the time might be spent just
transferring data. In this situation, a single HTTP transaction to a fast server could
easily consume all of the available modem bandwidth. If multiple objects are loaded
in parallel, each object will just compete for this limited bandwidth, so each object
will load proportionally slower, yielding little or no performance advantage.*
Also, a large number of open connections can consume a lot of memory and cause
performance problems of their own. Complex web pages may have tens or hundreds
of embedded objects. Clients might be able to open hundreds of connections, but
few web servers will want to do that, because they often are processing requests for
many other users at the same time. A hundred simultaneous users, each opening 100
connections, will put the burden of 10,000 connections on the server. This can cause
significant server slowdown. The same situation is true for high-load proxies.
In practice, browsers do use parallel connections, but they limit the total number of
parallel connections to a small number (often four). Servers are free to close exces-
sive connections from a particular client.
Parallel Connections May “Feel” Faster
Okay, so parallel connections don’t always make pages load faster. But even if they
don’t actually speed up the page transfer, as we said earlier, parallel connections
often make users feel that the page loads faster, because they can see progress being
made as multiple component objects appear onscreen in parallel.†Human beings
perceive that web pages load faster if there’s lots of action all over the screen, even if
a stopwatch actually shows the aggregate page download time to be slower!
Persistent Connections
Web clients often open connections to the same site. For example, most of the
embedded images in a web page often come from the same web site, and a signifi-
cant number of hyperlinks to other objects often point to the same site. Thus, an
application that initiates an HTTP request to a server likely will make more requests
to that server in the near future (to fetch the inline images, for example). This prop-
erty is called site locality.
For this reason, HTTP/1.1 (and enhanced versions of HTTP/1.0) allows HTTP
devices to keep TCP connections open after transactions complete and to reuse the
preexisting connections for future HTTP requests. TCP connections that are kept
* In fact, because of the extra overhead from multiple connections, it’s quite possible that parallel connections
could take longer to load the entire page than serial downloads.
† This effect is amplified by the increasing use of progressive images that produce low-resolution approxima-
tions of images first and gradually increase the resolution.
Persistent Connections |91
open after transactions complete are called persistent connections. Nonpersistent
connections are closed after each transaction. Persistent connections stay open
across transactions, until either the client or the server decides to close them.
By reusing an idle, persistent connection that is already open to the target server, you
can avoid the slow connection setup. In addition, the already open connection can
avoid the slow-start congestion adaptation phase, allowing faster data transfers.
Persistent Versus Parallel Connections
As we’ve seen, parallel connections can speed up the transfer of composite pages.
But parallel connections have some disadvantages:
• Each transaction opens/closes a new connection, costing time and bandwidth.
• Each new connection has reduced performance because of TCP slow start.
• There is a practical limit on the number of open parallel connections.
Persistent connections offer some advantages over parallel connections. They reduce
the delay and overhead of connection establishment, keep the connections in a tuned
state, and reduce the potential number of open connections. However, persistent
connections need to be managed with care, or you may end up accumulating a large
number of idle connections, consuming local resources and resources on remote cli-
ents and servers.
Persistent connections can be most effective when used in conjunction with parallel
connections. Today, many web applications open a small number of parallel connec-
tions, each persistent. There are two types of persistent connections: the older
HTTP/1.0+ “keep-alive” connections and the modern HTTP/1.1 “persistent” con-
nections. We’ll look at both flavors in the next few sections.
HTTP/1.0+ Keep-Alive Connections
Many HTTP/1.0 browsers and servers were extended (starting around 1996) to sup-
port an early, experimental type of persistent connections called keep-alive connec-
tions. These early persistent connections suffered from some interoperability design
problems that were rectified in later revisions of HTTP/1.1, but many clients and
servers still use these earlier keep-alive connections.
Some of the performance advantages of keep-alive connections are visible in
Figure 4-13, which compares the timeline for four HTTP transactions over serial con-
nections against the same transactions over a single persistent connection. The time-
line is compressed because the connect and close overheads are removed.*
* Additionally, the request and response time might also be reduced because of elimination of the slow-start
phase. This performance benefit is not depicted in the figure.
92 |Chapter 4: Connection Management
Keep-Alive Operation
Keep-alive is deprecated and no longer documented in the current HTTP/1.1 specifi-
cation. However, keep-alive handshaking is still in relatively common use by brows-
ers and servers, so HTTP implementors should be prepared to interoperate with it.
We’ll take a quick look at keep-alive operation now. Refer to older versions of the
HTTP/1.1 specification (such as RFC 2068) for a more complete explanation of
keep-alive handshaking.
Clients implementing HTTP/1.0 keep-alive connections can request that a connec-
tion be kept open by including the Connection: Keep-Alive request header.
If the server is willing to keep the connection open for the next request, it will
respond with the same header in the response (see Figure 4-14). If there is no Con-
nection: keep-alive header in the response, the client assumes that the server does
not support keep-alive and that the server will close the connection when the
response message is sent back.
Keep-Alive Options
Note that the keep-alive headers are just requests to keep the connection alive. Cli-
ents and servers do not need to agree to a keep-alive session if it is requested. They
Figure 4-13. Four transactions (serial versus persistent)
Client
Server
Transaction 1
Time
Transaction 4
Request- 1
Request- 2
Request- 3
Request- 4
Response- 1
Response- 2
Response- 3
Response- 4
Client
Server
Transaction 1
Time
Connect- 1 Connect- 2 Connect- 3 Connect- 4
Transaction 2 Transaction 3 Transaction 4
Request- 1
Request- 2
Request- 3
Request- 4
Response- 1
Response- 2
Response- 3
Response- 4
Transaction 2 Transaction 3
(a) Serial connections
(b) Persistent connection
Persistent Connections |93
can close idle keep-alive connections at any time and are free to limit the number of
transactions processed on a keep-alive connection.
The keep-alive behavior can be tuned by comma-separated options specified in the
Keep-Alive general header:
• The timeout parameter is sent in a Keep-Alive response header. It estimates how
long the server is likely to keep the connection alive for. This is not a guarantee.
• The max parameter is sent in a Keep-Alive response header. It estimates how
many more HTTP transactions the server is likely to keep the connection alive
for. This is not a guarantee.
• The Keep-Alive header also supports arbitrary unprocessed attributes, primarily
for diagnostic and debugging purposes. The syntax is name [= value].
The Keep-Alive header is completely optional but is permitted only when Connec-
tion: Keep-Alive also is present. Here’s an example of a Keep-Alive response header
indicating that the server intends to keep the connection open for at most five more
transactions, or until it has sat idle for two minutes:
Connection: Keep-Alive
Keep-Alive: max=5, timeout=120
Keep-Alive Connection Restrictions and Rules
Here are some restrictions and clarifications regarding the use of keep-alive
connections:
• Keep-alive does not happen by default in HTTP/1.0. The client must send a
Connection: Keep-Alive request header to activate keep-alive connections.
• The Connection: Keep-Alive header must be sent with all messages that want to
continue the persistence. If the client does not send a Connection: Keep-Alive
header, the server will close the connection after that request.
Figure 4-14. HTTP/1.0 keep-alive transaction header handshake
Internet
Client Server
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
Connection: Keep-Alive
HTTP/1.0 200 OK
Content-type: text/html
Content-length: 3104
Connection: Keep-Alive
...
94 |Chapter 4: Connection Management
• Clients can tell if the server will close the connection after the response by
detecting the absence of the Connection: Keep-Alive response header.
• The connection can be kept open only if the length of the message’s entity body
can be determined without sensing a connection close—this means that the entity
body must have a correct Content-Length, have a multipart media type, or be
encoded with the chunked transfer encoding. Sending the wrong Content-Length
back on a keep-alive channel is bad, because the other end of the transaction will
not be able to accurately detect the end of one message and the start of another.
• Proxies and gateways must enforce the rules of the Connection header; the proxy
or gateway must remove any header fields named in the Connection header, and
the Connection header itself, before forwarding or caching the message.
• Formally, keep-alive connections should not be established with a proxy server
that isn’t guaranteed to support the Connection header, to prevent the problem
with dumb proxies described below. This is not always possible in practice.
• Technically, any Connection header fields (including Connection: Keep-Alive)
received from an HTTP/1.0 device should be ignored, because they may have
been forwarded mistakenly by an older proxy server. In practice, some clients
and servers bend this rule, although they run the risk of hanging on older proxies.
• Clients must be prepared to retry requests if the connection closes before they
receive the entire response, unless the request could have side effects if repeated.
Keep-Alive and Dumb Proxies
Let’s take a closer look at the subtle problem with keep-alive and dumb proxies. A
web client’s Connection: Keep-Alive header is intended to affect just the single TCP
link leaving the client. This is why it is named the “connection” header. If the client
is talking to a web server, the client sends a Connection: Keep-Alive header to tell the
server it wants keep-alive. The server sends a Connection: Keep-Alive header back if
it supports keep-alive and doesn’t send it if it doesn’t.
The Connection header and blind relays
The problem comes with proxies—in particular, proxies that don’t understand the
Connection header and don’t know that they need to remove the header before proxy-
ing it down the chain. Many older or simple proxies act as blind relays, tunneling bytes
from one connection to another, without specially processing the Connection header.
Imagine a web client talking to a web server through a dumb proxy that is acting as a
blind relay. This situation is depicted in Figure 4-15.
Here’s what’s going on in this figure:
1. In Figure 4-15a, a web client sends a message to the proxy, including the Connec-
tion: Keep-Alive header, requesting a keep-alive connection if possible. The client
waits for a response to learn if its request for a keep-alive channel was granted.
Persistent Connections |95
2. The dumb proxy gets the HTTP request, but it doesn’t understand the Connec-
tion header (it just treats it as an extension header). The proxy has no idea what
keep-alive is, so it passes the message verbatim down the chain to the server
(Figure 4-15b). But the Connection header is a hop-by-hop header; it applies to
only a single transport link and shouldn’t be passed down the chain. Bad things
are about to happen.
3. In Figure 4-15b, the relayed HTTP request arrives at the web server. When the
web server receives the proxied Connection: Keep-Alive header, it mistakenly
concludes that the proxy (which looks like any other client to the server) wants
to speak keep-alive! That’s fine with the web server—it agrees to speak keep-
alive and sends a Connection: Keep-Alive response header back in Figure 4-15c.
So, at this point, the web server thinks it is speaking keep-alive with the proxy
and will adhere to rules of keep-alive. But the proxy doesn’t know the first thing
about keep-alive. Uh-oh.
4. In Figure 4-15d, the dumb proxy relays the web server’s response message back to
the client, passing along the Connection: Keep-Alive header from the web server.
The client sees this header and assumes the proxy has agreed to speak keep-alive.
So at this point, both the client and server believe they are speaking keep-alive,
but the proxy they are talking to doesn’t know anything about keep-alive.
5. Because the proxy doesn’t know anything about keep-alive, it reflects all the
data it receives back to the client and then waits for the origin server to close the
connection. But the origin server will not close the connection, because it
believes the proxy explicitly asked the server to keep the connection open. So
the proxy will hang waiting for the connection to close.
6. When the client gets the response message back in Figure 4-15d, it moves right
along to the next request, sending another request to the proxy on the keep-alive
connection (see Figure 4-15e). Because the proxy never expects another request
Figure 4-15. Keep-alive doesn’t interoperate with proxies that don’t support Connection headers
Client Server
Dumb proxy
(a) Connection: Keep-Alive (b) Connection: Keep-Alive
(d) Connection: Keep-Alive (c) Connection: Keep-Alive
Next request
(c) Proxy waits for connection
to close, ignoring any new
requests on the connection
(e) Client’s second request on
the keep-alive connection just
hangs because the proxy never
processes it
(b) Server won’t close connection
when done because it thinks it has
been asked to speak keep-alive
96 |Chapter 4: Connection Management
on the same connection, the request is ignored and the browser just spins, mak-
ing no progress.
7. This miscommunication causes the browser to hang until the client or server
times out the connection and closes it.*
Proxies and hop-by-hop headers
To avoid this kind of proxy miscommunication, modern proxies must never proxy
the Connection header or any headers whose names appear inside the Connection
values. So if a proxy receives a Connection: Keep-Alive header, it shouldn’t proxy
either the Connection header or any headers named Keep-Alive.
In addition, there are a few hop-by-hop headers that might not be listed as values of
a Connection header, but must not be proxied or served as a cache response either.
These include Proxy-Authenticate, Proxy-Connection, Transfer-Encoding, and
Upgrade. For more information, refer back to “The Oft-Misunderstood Connection
Header.”
The Proxy-Connection Hack
Browser and proxy implementors at Netscape proposed a clever workaround to the
blind relay problem that didn’t require all web applications to support advanced ver-
sions of HTTP. The workaround introduced a new header called Proxy-Connection
and solved the problem of a single blind relay interposed directly after the client—
but not all other situations. Proxy-Connection is implemented by modern browsers
when proxies are explicitly configured and is understood by many proxies.
The idea is that dumb proxies get into trouble because they blindly forward hop-by-
hop headers such as Connection: Keep-Alive. Hop-by-hop headers are relevant only
for that single, particular connection and must not be forwarded. This causes trou-
ble when the forwarded headers are misinterpreted by downstream servers as
requests from the proxy itself to control its connection.
In the Netscape workaround, browsers send nonstandard Proxy-Connection exten-
sion headers to proxies, instead of officially supported and well-known Connection
headers. If the proxy is a blind relay, it relays the nonsense Proxy-Connection header
to the web server, which harmlessly ignores the header. But if the proxy is a smart
proxy (capable of understanding persistent connection handshaking), it replaces the
nonsense Proxy-Connection header with a Connection header, which is then sent to
the server, having the desired effect.
Figure 4-16a–d shows how a blind relay harmlessly forwards Proxy-Connection head-
ers to the web server, which ignores the header, causing no keep-alive connection to
* There are many similar scenarios where failures occur due to blind relays and forwarded handshaking.
Persistent Connections |97
be established between the client and proxy or the proxy and server. The smart proxy
in Figure 4-16e–h understands the Proxy-Connection header as a request to speak
keep-alive, and it sends out its own Connection: Keep-Alive headers to establish
keep-alive connections.
This scheme works around situations where there is only one proxy between the cli-
ent and server. But if there is a smart proxy on either side of the dumb proxy, the
problem will rear its ugly head again, as shown in Figure 4-17.
Furthermore, it is becoming quite common for “invisible” proxies to appear in net-
works, either as firewalls, intercepting caches, or reverse proxy server accelerators.
Because these devices are invisible to the browser, the browser will not send them
Proxy-Connection headers. It is critical that transparent web applications implement
persistent connections correctly.
HTTP/1.1 Persistent Connections
HTTP/1.1 phased out support for keep-alive connections, replacing them with an
improved design called persistent connections. The goals of persistent connections are
the same as those of keep-alive connections, but the mechanisms behave better.
Figure 4-16. Proxy-Connection header fixes single blind relay
Client Server
Dumb proxy
(a) Proxy-Connection: Keep-Alive (b) Proxy-Connection: Keep-Alive
(d) No Connection header (c) No Connection header
A dumb proxy forwards the Proxy-Connection header, which the server ignores.
The proxy recognizes the Proxy-Connection header, agrees to talk
keep-alive with the client, and may also (optionally) decide to
set up a keep-alive Connection with the server.
Client Server
Smart proxy
(e) Proxy-Connection: Keep-Alive (f) Connection: Keep-Alive
(h) Connection: Keep-Alive (g) Connection: Keep-Alive
A smart proxy understands the Proxy-Connection header and actively sends
a Connection: Keep-Alive header to the server.
The server does not recognize the Proxy-Connection header, and ignores it.
No keep-alive Connection is established.
98 |Chapter 4: Connection Management
Unlike HTTP/1.0+ keep-alive connections, HTTP/1.1 persistent connections are
active by default. HTTP/1.1 assumes all connections are persistent unless otherwise
indicated. HTTP/1.1 applications have to explicitly add a Connection: close header
to a message to indicate that a connection should close after the transaction is com-
plete. This is a significant difference from previous versions of the HTTP protocol,
where keep-alive connections were either optional or completely unsupported.
An HTTP/1.1 client assumes an HTTP/1.1 connection will remain open after a
response, unless the response contains a Connection: close header. However, clients
and servers still can close idle connections at any time. Not sending Connection:
close does not mean that the server promises to keep the connection open forever.
Persistent Connection Restrictions and Rules
Here are the restrictions and clarifications regarding the use of persistent connections:
• After sending a Connection: close request header, the client can’t send more
requests on that connection.
• If a client does not want to send another request on the connection, it should
send a Connection: close request header in the final request.
• The connection can be kept persistent only if all messages on the connection
have a correct, self-defined message length—i.e., the entity bodies must have
correct Content-Lengths or be encoded with the chunked transfer encoding.
Figure 4-17. Proxy-Connection still fails for deeper hierarchies of proxies
Client Server
Dumb
proxy
(a)
Proxy-Connection: Keep-Alive
(f)
Connection: Keep-Alive
A dumb proxy unwittingly advertises keep-Alive to browser and smart proxy.
Smart
proxy
(b)
Proxy-Connection: Keep-Alive
(e)
Connection: Keep-Alive
(c)
Connection: Keep-Alive
(d)
Connection: Keep-Alive
Client Server
Smart
proxy
(g)
Proxy-Connection: Keep-Alive
(l)
Connection: Keep-Alive
A dumb proxy unwittingly advertises keep-Alive to smart proxy and server.
Dumb
proxy
(h)
Connection: Keep-Alive
(k)
Connection: Keep-Alive
(i)
Connection: Keep-Alive
(j)
Connection: Keep-Alive
Pipelined Connections |99
• HTTP/1.1 proxies must manage persistent connections separately with clients
and servers—each persistent connection applies to a single transport hop.
• HTTP/1.1 proxy servers should not establish persistent connections with an
HTTP/1.0 client (because of the problems of older proxies forwarding Connec-
tion headers) unless they know something about the capabilities of the client.
This is, in practice, difficult, and many vendors bend this rule.
• Regardless of the values of Connection headers, HTTP/1.1 devices may close the
connection at any time, though servers should try not to close in the middle of
transmitting a message and should always respond to at least one request before
closing.
• HTTP/1.1 applications must be able to recover from asynchronous closes. Cli-
ents should retry the requests as long as they don’t have side effects that could
accumulate.
• Clients must be prepared to retry requests if the connection closes before they
receive the entire response, unless the request could have side effects if repeated.
• A single user client should maintain at most two persistent connections to any
server or proxy, to prevent the server from being overloaded. Because proxies
may need more connections to a server to support concurrent users, a proxy
should maintain at most 2Nconnections to any server or parent proxy, if there
are N users trying to access the servers.
Pipelined Connections
HTTP/1.1 permits optional request pipelining over persistent connections. This is a
further performance optimization over keep-alive connections. Multiple requests can
be enqueued before the responses arrive. While the first request is streaming across
the network to a server on the other side of the globe, the second and third requests
can get underway. This can improve performance in high-latency network condi-
tions, by reducing network round trips.
Figure 4-18a-c shows how persistent connections can eliminate TCP connection
delays and how pipelined requests (Figure 4-18c) can eliminate transfer latencies.
There are several restrictions for pipelining:
• HTTP clients should not pipeline until they are sure the connection is persistent.
• HTTP responses must be returned in the same order as the requests. HTTP mes-
sages are not tagged with sequence numbers, so there is no way to match
responses with requests if the responses are received out of order.
• HTTP clients must be prepared for the connection to close at any time and be
prepared to redo any pipelined requests that did not finish. If the client opens a
persistent connection and immediately issues 10 requests, the server is free to
close the connection after processing only, say, 5 requests. The remaining 5
100 |Chapter 4: Connection Management
requests will fail, and the client must be willing to handle these premature closes
and reissue the requests.
• HTTP clients should not pipeline requests that have side effects (such as
POSTs). In general, on error, pipelining prevents clients from knowing which of
a series of pipelined requests were executed by the server. Because nonidempo-
tent requests such as POSTs cannot safely be retried, you run the risk of some
methods never being executed in error conditions.
Figure 4-18. Four transactions (pipelined connections)
Request- 1
Resp 1
Client
Server
Transaction 1
Time
Transaction 4
Request- 1
Request- 2
Request- 3
Request- 4
Response- 1
Response- 2
Response- 3
Response- 4
Client
Server
Transaction 1
Time
Connect- 1 Connect- 2 Connect- 3 Connect- 4
Transaction 2 Transaction 3 Transaction 4
Request- 1
Request- 2
Request- 3
Request- 4
Response- 1
Response- 2
Response- 3
Response- 4
Transaction 2 Transaction 3
(a) Serial connections
(b) Persistent connection
Client
Server
Time
Transaction- 1
Request- 2
Request- 3
Re e- 2
Response- 3
Response- 4
Request- 4
Transaction- 2
Transaction- 3
Transaction- 4
(c) Pipelined, persistent connection
The Mysteries of Connection Close |101
The Mysteries of Connection Close
Connection management—particularly knowing when and how to close connec-
tions—is one of the practical black arts of HTTP. This issue is more subtle than
many developers first realize, and little has been written on the subject.
“At Will” Disconnection
Any HTTP client, server, or proxy can close a TCP transport connection at any time.
The connections normally are closed at the end of a message,*but during error con-
ditions, the connection may be closed in the middle of a header line or in other
strange places.
This situation is common with pipelined persistent connections. HTTP applications
are free to close persistent connections after any period of time. For example, after a
persistent connection has been idle for a while, a server may decide to shut it down.
However, the server can never know for sure that the client on the other end of the
line wasn’t about to send data at the same time that the “idle” connection was being
shut down by the server. If this happens, the client sees a connection error in the
middle of writing its request message.
Content-Length and Truncation
Each HTTP response should have an accurate Content-Length header to describe the
size of the response body. Some older HTTP servers omit the Content-Length header
or include an erroneous length, depending on a server connection close to signify the
actual end of data.
When a client or proxy receives an HTTP response terminating in connection close,
and the actual transferred entity length doesn’t match the Content-Length (or there
is no Content-Length), the receiver should question the correctness of the length.
If the receiver is a caching proxy, the receiver should not cache the response (to mini-
mize future compounding of a potential error). The proxy should forward the ques-
tionable message intact, without attempting to “correct” the Content-Length, to
maintain semantic transparency.
Connection Close Tolerance, Retries, and Idempotency
Connections can close at any time, even in non-error conditions. HTTP applica-
tions have to be ready to properly handle unexpected closes. If a transport connec-
tion closes while the client is performing a transaction, the client should reopen the
* Servers shouldn’t close a connection in the middle of a response unless client or network failure is suspected.
102 |Chapter 4: Connection Management
connection and retry one time, unless the transaction has side effects. The situation
is worse for pipelined connections. The client can enqueue a large number of
requests, but the origin server can close the connection, leaving numerous requests
unprocessed and in need of rescheduling.
Side effects are important. When a connection closes after some request data was
sent but before the response is returned, the client cannot be 100% sure how much
of the transaction actually was invoked by the server. Some transactions, such as
GETting a static HTML page, can be repeated again and again without changing
anything. Other transactions, such as POSTing an order to an online book store,
shouldn’t be repeated, or you may risk multiple orders.
A transaction is idempotent if it yields the same result regardless of whether it is exe-
cuted once or many times. Implementors can assume the GET, HEAD, PUT,
DELETE, TRACE, and OPTIONS methods share this property.*Clients shouldn’t
pipeline nonidempotent requests (such as POSTs). Otherwise, a premature termina-
tion of the transport connection could lead to indeterminate results. If you want to
send a nonidempotent request, you should wait for the response status for the previ-
ous request.
Nonidempotent methods or sequences must not be retried automatically, although
user agents may offer a human operator the choice of retrying the request. For exam-
ple, most browsers will offer a dialog box when reloading a cached POST response,
asking if you want to post the transaction again.
Graceful Connection Close
TCP connections are bidirectional, as shown in Figure 4-19. Each side of a TCP con-
nection has an input queue and an output queue, for data being read or written.
Data placed in the output of one side will eventually show up on the input of the
other side.
Full and half closes
An application can close either or both of the TCP input and output channels. A
close( ) sockets call closes both the input and output channels of a TCP connection.
*Administrators who use GET-based dynamic forms should make sure the forms are idempotent.
Figure 4-19. TCP connections are bidirectional
Client Server
in out
inout
The Mysteries of Connection Close |103
This is called a “full close” and is depicted in Figure 4-20a. You can use the
shutdown( ) sockets call to close either the input or output channel individually. This
is called a “half close” and is depicted in Figure 4-20b.
TCP close and reset errors
Simple HTTP applications can use only full closes. But when applications start talk-
ing to many other types of HTTP clients, servers, and proxies, and when they start
using pipelined persistent connections, it becomes important for them to use half
closes to prevent peers from getting unexpected write errors.
In general, closing the output channel of your connection is always safe. The peer on
the other side of the connection will be notified that you closed the connection by
getting an end-of-stream notification once all the data has been read from its buffer.
Closing the input channel of your connection is riskier, unless you know the other
side doesn’t plan to send any more data. If the other side sends data to your closed
input channel, the operating system will issue a TCP “connection reset by peer” mes-
sage back to the other side’s machine, as shown in Figure 4-21. Most operating sys-
tems treat this as a serious error and erase any buffered data the other side has not
read yet. This is very bad for pipelined connections.
Say you have sent 10 pipelined requests on a persistent connection, and the
responses already have arrived and are sitting in your operating system’s buffer (but
the application hasn’t read them yet). Now say you send request #11, but the server
decides you’ve used this connection long enough, and closes it. Your request #11
will arrive at a closed connection and will reflect a reset back to you. This reset will
erase your input buffers.
Figure 4-20. Full and half close
Client Server
in out
inout
(a) Server full close
Client Server
in out
inout
(b) Server output half close (graceful close)
Client Server
in out
inout
(c) Server input half close
104 |Chapter 4: Connection Management
When you finally get to reading data, you will get a connection reset by peer error,
and the buffered, unread response data will be lost, even though much of it success-
fully arrived at your machine.
Graceful close
The HTTP specification counsels that when clients or servers want to close a connec-
tion unexpectedly, they should “issue a graceful close on the transport connection,”
but it doesn’t describe how to do that.
In general, applications implementing graceful closes will first close their output
channels and then wait for the peer on the other side of the connection to close its
output channels. When both sides are done telling each other they won’t be sending
any more data (i.e., closing output channels), the connection can be closed fully,
with no risk of reset.
Unfortunately, there is no guarantee that the peer implements or checks for half
closes. For this reason, applications wanting to close gracefully should half close
their output channels and periodically check the status of their input channels (look-
ing for data or for the end of the stream). If the input channel isn’t closed by the peer
within some timeout period, the application may force connection close to save
resources.
For More Information
This completes our overview of the HTTP plumbing trade. Please refer to the fol-
lowing reference sources for more information about TCP performance and HTTP
connection-management facilities.
HTTP Connections
http://www.ietf.org/rfc/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol—HTTP/1.1,” is the official specification
for HTTP/1.1; it explains the usage of and HTTP header fields for implementing
Figure 4-21. Data arriving at closed connection generates “connection reset by peer” error
Client Server
in out
inout
RESET
For More Information |105
parallel, persistent, and pipelined HTTP connections. This document does not
cover the proper use of the underlying TCP connections.
http://www.ietf.org/rfc/rfc2068.txt
RFC 2068 is the 1997 version of the HTTP/1.1 protocol. It contains explanation
of the HTTP/1.0+ Keep-Alive connections that is missing from RFC 2616.
http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt
This expired Internet draft, “HTTP Connection Management,” has some good
discussion of issues facing HTTP connection management.
HTTP Performance Issues
http://www.w3.org/Protocols/HTTP/Performance/
This W3C web page, entitled “HTTP Performance Overview,” contains a few
papers and tools related to HTTP performance and connection management.
http://www.w3.org/Protocols/HTTP/1.0/HTTPPerformance.html
This short memo by Simon Spero, “Analysis of HTTP Performance Problems,” is
one of the earliest (1994) assessments of HTTP connection performance. The
memo gives some early performance measurements of the effect of connection
setup, slow start, and lack of connection sharing.
ftp://gatekeeper.dec.com/pub/DEC/WRL/research-reports/WRL-TR-95.4.pdf
“The Case for Persistent-Connection HTTP.”
http://www.isi.edu/lsam/publications/phttp_tcp_interactions/paper.html
“Performance Interactions Between P-HTTP and TCP Implementations.”
http://www.sun.com/sun-on-net/performance/tcp.slowstart.html
“TCP Slow Start Tuning for Solaris” is a web page from Sun Microsystems that
talks about some of the practical implications of TCP slow start. It’s a useful
read, even if you are working with different operating systems.
TCP/IP
The following three books by W. Richard Stevens are excellent, detailed engineering
texts on TCP/IP. These are extremely useful for anyone using TCP:
TCP Illustrated, Volume I: The Protocols
W. Richard Stevens, Addison Wesley
UNIX Network Programming, Volume 1: Networking APIs
W. Richard Stevens, Prentice-Hall
UNIX Network Programming, Volume 2: The Implementation
W. Richard Stevens, Prentice-Hall
106 |Chapter 4: Connection Management
The following papers and specifications describe TCP/IP and features that affect its
performance. Some of these specifications are over 20 years old and, given the world-
wide success of TCP/IP, probably can be classified as historical treasures:
http://www.acm.org/sigcomm/ccr/archive/2001/jan01/ccr-200101-mogul.pdf
In “Rethinking the TCP Nagle Algorithm,” Jeff Mogul and Greg Minshall
present a modern perspective on Nagle’s algorithm, outline what applications
should and should not use the algorithm, and propose several modifications.
http://www.ietf.org/rfc/rfc2001.txt
RFC 2001, “TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast
Recovery Algorithms,” defines the TCP slow-start algorithm.
http://www.ietf.org/rfc/rfc1122.txt
RFC 1122, “Requirements for Internet Hosts—Communication Layers,” dis-
cusses TCP acknowledgment and delayed acknowledgments.
http://www.ietf.org/rfc/rfc896.txt
RFC 896, “Congestion Control in IP/TCP Internetworks,” was released by John
Nagle in 1984. It describes the need for TCP congestion control and introduces
what is now called “Nagle’s algorithm.”
http://www.ietf.org/rfc/rfc0813.txt
RFC 813, “Window and Acknowledgement Strategy in TCP,” is a historical
(1982) specification that describes TCP window and acknowledgment imple-
mentation strategies and provides an early description of the delayed acknowl-
edgment technique.
http://www.ietf.org/rfc/rfc0793.txt
RFC 793, “Transmission Control Protocol,” is Jon Postel’s classic 1981 defini-
tion of the TCP protocol.
PART II
HTTP Architecture
The six chapters of Part II highlight the HTTP server, proxy, cache, gateway, and
robot applications, which are the building blocks of web systems architecture:
• Chapter 5, Web Servers, gives an overview of web server architectures.
• Chapter 6, Proxies, describes HTTP proxy servers, which are intermediary servers
that connect HTTP clients and act as platforms for HTTP services and controls.
• Chapter 7, Caching, delves into the science of web caches—devices that improve
performance and reduce traffic by making local copies of popular documents.
• Chapter 8, Integration Points: Gateways, Tunnels, and Relays, explains applica-
tions that allow HTTP to interoperate with software that speaks different proto-
cols, including SSL encrypted protocols.
• Chapter 9, Web Robots, wraps up our tour of HTTP architecture with web clients.
• Chapter 10, HTTP-NG, covers future topics for HTTP—in particular, HTTP-NG.
109
CHAPTER 5
Web Servers
Web servers dish out billions of web pages a day. They tell you the weather, load up
your online shopping carts, and let you find long-lost high-school buddies. Web
servers are the workhorses of the World Wide Web. In this chapter, we:
• Survey the many different types of software and hardware web servers.
• Describe how to write a simple diagnostic web server in Perl.
• Explain how web servers process HTTP transactions, step by step.
Where it helps to make things concrete, our examples use the Apache web server and
its configuration options.
Web Servers Come in All Shapes and Sizes
A web server processes HTTP requests and serves responses. The term “web server”
can refer either to web server software or to the particular device or computer dedi-
cated to serving the web pages.
Web servers comes in all flavors, shapes, and sizes. There are trivial 10-line Perl
script web servers, 50-MB secure commerce engines, and tiny servers-on-a-card. But
whatever the functional differences, all web servers receive HTTP requests for
resources and serve content back to the clients (look back to Figure 1-5).
Web Server Implementations
Web servers implement HTTP and the related TCP connection handling. They also
manage the resources served by the web server and provide administrative features to
configure, control, and enhance the web server.
The web server logic implements the HTTP protocol, manages web resources, and
provides web server administrative capabilities. The web server logic shares responsi-
bilities for managing TCP connections with the operating system. The underlying
110 |Chapter 5: Web Servers
operating system manages the hardware details of the underlying computer system
and provides TCP/IP network support, filesystems to hold web resources, and pro-
cess management to control current computing activities.
Web servers are available in many forms:
• You can install and run general-purpose software web servers on standard com-
puter systems.
• If you don’t want the hassle of installing software, you can purchase a web server
appliance, in which the software comes preinstalled and preconfigured on a
computer, often in a snazzy-looking chassis.
• Given the miracles of microprocessors, some companies even offer embedded
web servers implemented in a small number of computer chips, making them
perfect administration consoles for consumer devices.
Let’s look at each of those types of implementations.
General-Purpose Software Web Servers
General-purpose software web servers run on standard, network-enabled computer
systems. You can choose open source software (such as Apache or W3C’s Jigsaw) or
commercial software (such as Microsoft’s and iPlanet’s web servers). Web server
software is available for just about every computer and operating system.
While there are tens of thousands of different kinds of web server programs (includ-
ing custom-crafted, special-purpose web servers), most web server software comes
from a small number of organizations.
In February 2002, the Netcraft survey (http://www.netcraft.com/survey/) showed three
vendors dominating the public Internet web server market (see Figure 5-1):
• The free Apache software powers nearly 60% of all Internet web servers.
• Microsoft web server makes up another 30%.
• Sun iPlanet servers comprise another 3%.
Figure 5-1. Web server market share as estimated by Netcraft’s automated survey
A Minimal Perl Web Server |111
Take these numbers with a few grains of salt, however, as the Netcraft survey is com-
monly believed to exaggerate the dominance of Apache software. First, the survey
counts servers independent of server popularity. Proxy server access studies from
large ISPs suggest that the amount of pages served from Apache servers is much less
than 60% but still exceeds Microsoft and Sun iPlanet. Additionally, it is anecdotally
believed that Microsoft and iPlanet servers are more popular than Apache inside cor-
porate enterprises.
Web Server Appliances
Web server appliances are prepackaged software/hardware solutions. The vendor pre-
installs a software server onto a vendor-chosen computer platform and preconfig-
ures the software. Some examples of web server appliances include:
• Sun/Cobalt RaQ web appliances (http://www.cobalt.com)
• Toshiba Magnia SG10 (http://www.toshiba.com)
• IBM Whistle web server appliance (http://www.whistle.com)
Appliance solutions remove the need to install and configure software and often
greatly simplify administration. However, the web server often is less flexible and
feature-rich, and the server hardware is not easily repurposeable or upgradable.
Embedded Web Servers
Embedded servers are tiny web servers intended to be embedded into consumer prod-
ucts (e.g., printers or home appliances). Embedded web servers allow users to
administer their consumer devices using a convenient web browser interface.
Some embedded web servers can even be implemented in less than one square inch,
but they usually offer a minimal feature set. Two examples of very small embedded
web servers are:
• IPic match-head sized web server (http://www-ccs.cs.umass.edu/~shri/iPic.html)
• NetMedia SitePlayer SP1 Ethernet Web Server (http://www.siteplayer.com)
A Minimal Perl Web Server
If you want to build a full-featured HTTP server, you have some work to do. The
core of the Apache web server has over 50,000 lines of code, and optional processing
modules make that number much bigger.
All this software is needed to support HTTP/1.1 features: rich resource support, vir-
tual hosting, access control, logging, configuration, monitoring, and performance
features. That said, you can create a minimally functional HTTP server in under 30
lines of Perl. Let’s take a look.
112 |Chapter 5: Web Servers
Example 5-1 shows a tiny Perl program called type-o-serve. This program is a useful
diagnostic tool for testing interactions with clients and proxies. Like any web server,
type-o-serve waits for an HTTP connection. As soon as type-o-serve gets the request
message, it prints the message on the screen; then it waits for you to type (or paste)
in a response message, which is sent back to the client. This way, type-o-serve pre-
tends to be a web server, records the exact HTTP request messages, and allows you
to send back any HTTP response message.
This simple type-o-serve utility doesn’t implement most HTTP functionality, but it is
a useful tool to generate server response messages the same way you can use Telnet
to generate client request messages (refer back to Example 5-1). You can download
the type-o-serve program from http://www.http-guide.com/tools/type-o-serve.pl.
Example 5-1. type-o-serve—a minimal Perl web server used for HTTP debugging
#!/usr/bin/perl
use Socket;
use Carp;
use FileHandle;
# (1) use port 8080 by default, unless overridden on command line
$port = (@ARGV ? $ARGV[0] : 8080);
# (2) create local TCP socket and set it to listen for connections
$proto = getprotobyname('tcp');
socket(S, PF_INET, SOCK_STREAM, $proto) || die;
setsockopt(S, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) || die;
bind(S, sockaddr_in($port, INADDR_ANY)) || die;
listen(S, SOMAXCONN) || die;
# (3) print a startup message
printf(" <<<Type-O-Serve Accepting on Port %d>>>\n\n",$port);
while (1)
{
# (4) wait for a connection C
$cport_caddr = accept(C, S);
($cport,$caddr) = sockaddr_in($cport_caddr);
C->autoflush(1);
# (5) print who the connection is from
$cname = gethostbyaddr($caddr,AF_INET);
printf(" <<<Request From '%s'>>>\n",$cname);
# (6) read request msg until blank line, and print on screen
while ($line = <C>)
{
print $line;
if ($line =~ /^\r/) { last; }
}
What Real Web Servers Do |113
Figure 5-2 shows how the administrator of Joe’s Hardware store might use type-o-
serve to test HTTP communication:
• First, the administrator starts the type-o-serve diagnostic server, listening on a
particular port. Because Joe’s Hardware store already has a production web
server listing on port 80, the administrator starts the type-o-serve server on port
8080 (you can pick any unused port) with this command line:
%type-o-serve.pl 8080
• Once type-o-serve is running, you can point a browser to this web server. In
Figure 5-2, we browse to http://www.joes-hardware.com:8080/foo/bar/blah.txt.
• The type-o-serve program receives the HTTP request message from the browser
and prints the contents of the HTTP request message on screen. The type-o-serve
diagnostic tool then waits for the user to type in a simple response message, fol-
lowed by a period on a blank line.
•type-o-serve sends the HTTP response message back to the browser, and the
browser displays the body of the response message.
What Real Web Servers Do
The Perl server we showed in Example 5-1 is a trivial example web server. State-of-
the-art commercial web servers are much more complicated, but they do perform
several common tasks, as shown in Figure 5-3:
1. Set up connection—accept a client connection, or close if the client is unwanted.
2. Receive request—read an HTTP request message from the network.
3. Process request—interpret the request message and take action.
4. Access resource—access the resource specified in the message.
5. Construct response—create the HTTP response message with the right headers.
6. Send response—send the response back to the client.
7. Log transaction—place notes about the completed transaction in a log file.
# (7) prompt for response message, and input response lines,
# sending response lines to client, until solitary "."
printf(" <<<Type Response Followed by '.'>>>\n");
while ($line = <STDIN>)
{
$line =~ s/\r//;
$line =~ s/\n//;
if ($line =~ /^\./) { last; }
print C $line . "\r\n";
}
close(C);
}
Example 5-1. type-o-serve—a minimal Perl web server used for HTTP debugging (continued)
114 |Chapter 5: Web Servers
The next seven sections highlight how web servers perform these basic tasks.
Figure 5-2. The type-o-serve utility lets you type in server responses to send back to clients
Figure 5-3. Steps of a basic web server request
GET /foo/bar/blah.txt HTTP/1.1
Accept: */*
Accept-language: en-us
Accept-encoding: gzip, deflate
User-agent: Mozilla/4.0
Host: www.joes.hardware.com:8080
Connection: Keep-alive
HTTP request message
HTTP/1.0 200 OK
Connection: close
Content-type: text/plain
Hi there!
HTTP response message
% ./type-o-serve.pl 8080
<<<Type-O-Serve Accepting on Port 8080>>>
<<<Request From 'home-44-027.extranet.inktomi.com'>>>
GET /foo/bar/blah.txt HTTP/1.1
Accept: */*
Accept-language: en-us
Accept-encoding: gzip, deflate
User-agent: Mozilla/4.0
Host: www.joes-hardware.com:8080
Connection: Keep-alive
<<<Type response followed by '.'>>>
HTTP/1.0 200 OK
Connection: close
Content-type: text-plain
Hi there!
type-o-serve dialog
HTTP server software process
TCP/IP
network
stack
Client
Network interface Object storage
User space
Operating system
(2) Receive request
(3) Process request
(4) Access resource
(5) Create response
(7) Log transaction
(6) Send response
(1) Set up connection
Step 1: Accepting Client Connections |115
Step 1: Accepting Client Connections
If a client already has a persistent connection open to the server, it can use that connec-
tion to send its request. Otherwise, the client needs to open a new connection to the
server (refer back to Chapter 4 to review HTTP connection-management technology).
Handling New Connections
When a client requests a TCP connection to the web server, the web server estab-
lishes the connection and determines which client is on the other side of the connec-
tion, extracting the IP address from the TCP connection.*Once a new connection is
established and accepted, the server adds the new connection to its list of existing
web server connections and prepares to watch for data on the connection.
The web server is free to reject and immediately close any connection. Some web
servers close connections because the client IP address or hostname is unauthorized
or is a known malicious client. Other identification techniques can also be used.
Client Hostname Identification
Most web servers can be configured to convert client IP addresses into client host-
names, using “reverse DNS.” Web servers can use the client hostname for detailed
access control and logging. Be warned that hostname lookups can take a very long
time, slowing down web transactions. Many high-capacity web servers either disable
hostname resolution or enable it only for particular content.
You can enable hostname lookups in Apache with the HostnameLookups configura-
tion directive. For example, the Apache configuration directives in Example 5-2 turn
on hostname resolution for only HTML and CGI resources.
Determining the Client User Through ident
Some web servers also support the IETF ident protocol. The ident protocol lets
servers find out what username initiated an HTTP connection. This information is
* Different operating systems have different interfaces and data structures for manipulating TCP connections.
In Unix environments, the TCP connection is represented by a socket, and the IP address of the client can be
found from the socket using the getpeername call.
Example 5-2. Configuring Apache to look up hostnames for HTML and CGI resources
HostnameLookups off
<Files ~ "\.(html|htm|cgi)$">
HostnameLookups on
</Files>
116 |Chapter 5: Web Servers
particularly useful for web server logging—the second field of the popular Com-
mon Log Format contains the ident username of each HTTP request.*
If a client supports the ident protocol, the client listens on TCP port 113 for ident
requests. Figure 5-4 shows how the ident protocol works. In Figure 5-4a, the client
opens an HTTP connection. The server then opens its own connection back to the
client’s identd server port (113), sends a simple request asking for the username cor-
responding to the new connection (specified by client and server port numbers), and
retrieves from the client the response containing the username.
ident can work inside organizations, but it does not work well across the public Inter-
net for many reasons, including:
• Many client PCs don’t run the identd Identification Protocol daemon software.
• The ident protocol significantly delays HTTP transactions.
• Many firewalls won’t permit incoming ident traffic.
• The ident protocol is insecure and easy to fabricate.
• The ident protocol doesn’t support virtual IP addresses well.
• There are privacy concerns about exposing client usernames.
You can tell Apache web servers to use ident lookups with Apache’s IdentityCheck on
directive. If no ident information is available, Apache will fill ident log fields with
hyphens (-). Common Log Format log files typically contain hyphens in the second
field because no ident information is available.
Step 2: Receiving Request Messages
As the data arrives on connections, the web server reads out the data from the net-
work connection and parses out the pieces of the request message (Figure 5-5).
* This Common Log Format ident field is called “rfc931,” after an outdated version of the RFC defining the
ident protocol (the updated ident specification is documented by RFC 1413).
Figure 5-4. Using the ident protocol to determine HTTP client username
Mary Web server
HTTP connection
Port 4236 Port 80
(a) Mary establishes new HTTP connection
ident connection
Port 113
Port 80
(b) Server establishes ident connection
4236,80
4236,80: USERID: UNIX: mary
(d) Client returns ident response
(c) Server sends request
Step 2: Receiving Request Messages |117
When parsing the request message, the web server:
• Parses the request line looking for the request method, the specified resource
identifier (URI), and the version number,*each separated by a single space, and
ending with a carriage-return line-feed (CRLF) sequence†
• Reads the message headers, each ending in CRLF
• Detects the end-of-headers blank line, ending in CRLF (if present)
• Reads the request body, if any (length specified by the Content-Length header)
When parsing request messages, web servers receive input data erratically from the
network. The network connection can stall at any point. The web server needs to
read data from the network and temporarily store the partial message data in mem-
ory until it receives enough data to parse it and make sense of it.
Internal Representations of Messages
Some web servers also store the request messages in internal data structures that
make the message easy to manipulate. For example, the data structure might con-
tain pointers and lengths of each piece of the request message, and the headers might
be stored in a fast lookup table so the specific values of particular headers can be
accessed quickly (Figure 5-6).
Connection Input/Output Processing Architectures
High-performance web servers support thousands of simultaneous connections.
These connections let the web server communicate with clients around the world,
each with one or more connections open to the server. Some of these connections
may be sending requests rapidly to the web server, while other connections trickle
Figure 5-5. Reading a request message from a connection
* The initial version of HTTP, called HTTP/0.9, does not support version numbers. Some web servers support
missing version numbers, interpreting the message as an HTTP/0.9 request.
† Many web servers support LF or CRLF as end-of-line sequences, because some clients mistakenly send LF
as the end-of-line terminator.
Client Server
Internet
LF CR LF CR moc.erawdrah-seo
Request message being read from network
GET /specials/saw-blade.gif HTTP/1.0CRLF
Accept: image/gifCRLF
Host: www.j
118 |Chapter 5: Web Servers
requests slowly or infrequently, and still others are idle, waiting quietly for some
future activity.
Web servers constantly watch for new web requests, because requests can arrive at
any time. Different web server architectures service requests in different ways, as
Figure 5-7 illustrates:
Single-threaded web servers (Figure 5-7a)
Single-threaded web servers process one request at a time until completion.
When the transaction is complete, the next connection is processed. This archi-
tecture is simple to implement, but during processing, all the other connections
are ignored. This creates serious performance problems and is appropriate only
for low-load servers and diagnostic tools like type-o-serve.
Multiprocess and multithreaded web servers (Figure 5-7b)
Multiprocess and multithreaded web servers dedicate multiple processes or
higher-efficiency threads to process requests simultaneously.*The threads/
processes may be created on demand or in advance.†Some servers dedicate a
thread/process for every connection, but when a server processes hundreds,
thousands, or even tens of thousands of simultaneous connections, the resulting
number of processes or threads may consume too much memory or system
Figure 5-6. Parsing a request message into a convenient internal representation
* A process is an individual program flow of control, with its own set of variables. A thread is a faster, more
efficient version of a process. Both threads and processes let a single program do multiple things at the same
time. For simplicity of explanation, we treat processes and threads interchangeably. But, because of the per-
formance differences, many high-performance servers are both multiprocess and multithreaded.
† Systems where threads are created in advance are called “worker pool” systems, because a set of threads
waits in a pool for work to do.
GET /specials/saw-blade.gif HTTP/1.0CRLF
Accept: image/gifCRLF
Host: www.joes-hardware.comCRLF
CRLF
Request message
Parse
method: 1
version: 1.0
uri:
header count: 2
headers:
body: -
Parsed encoding of request message
specials/saw-blade.gif
www.joes-hardware.com
image/gif
name: Host
name: Accept
value:
value:
Step 2: Receiving Request Messages |119
resources. Thus, many multithreaded web servers put a limit on the maximum
number of threads/processes.
Multiplexed I/O servers (Figure 5-7c)
To support large numbers of connections, many web servers adopt multiplexed
architectures. In a multiplexed architecture, all the connections are simulta-
neously watched for activity. When a connection changes state (e.g., when data
becomes available or an error condition occurs), a small amount of processing is
performed on the connection; when that processing is complete, the connection
is returned to the open connection list for the next change in state. Work is done
on a connection only when there is something to be done; threads and processes
are not tied up waiting on idle connections.
Multiplexed multithreaded web servers (Figure 5-7d)
Some systems combine multithreading and multiplexing to take advantage of
multiple CPUs in the computer platform. Multiple threads (often one per physi-
cal processor) each watch the open connections (or a subset of the open connec-
tions) and perform a small amount of work on each connection.
Figure 5-7. Web server input/output architectures
(a) Single-threaded I/O architecture (b) Multithreaded I/O architecture
Connection list
Connection
Thread/process
(d) Multiplexed, multithreaded I/O architecture(c) Multiplexed I/O architecture
Connection
multiplexer
120 |Chapter 5: Web Servers
Step 3: Processing Requests
Once the web server has received a request, it can process the request using the
method, resource, headers, and optional body.
Some methods (e.g., POST) require entity body data in the request message. Other
methods (e.g., OPTIONS) allow a request body but don’t require one. A few meth-
ods (e.g., GET) forbid entity body data in request messages.
We won’t talk about request processing here, because it’s the subject of most of the
chapters in the rest of this book!
Step 4: Mapping and Accessing Resources
Web servers are resource servers. They deliver precreated content, such as HTML
pages or JPEG images, as well as dynamic content from resource-generating applica-
tions running on the servers.
Before the web server can deliver content to the client, it needs to identify the source
of the content, by mapping the URI from the request message to the proper content
or content generator on the web server.
Docroots
Web servers support different kinds of resource mapping, but the simplest form of
resource mapping uses the request URI to name a file in the web server’s filesystem.
Typically, a special folder in the web server filesystem is reserved for web content.
This folder is called the document root,ordocroot. The web server takes the URI
from the request message and appends it to the document root.
In Figure 5-8, a request arrives for /specials/saw-blade.gif. The web server in this
example has document root /usr/local/httpd/files. The web server returns the file /usr/
local/httpd/files/specials/saw-blade.gif.
Figure 5-8. Mapping request URI to local web server resource
Internet
Request message
GET /specials/saw-blade.gif HTTP/1.0
Host: www.joes-hardware.com
Client
/usr/local/httpd/files
Web server
object storage
Request URI: /specials/saw-blade.gif Server resource: /usr/local/httpd/files/specials/saw-blade.gif
Step 4: Mapping and Accessing Resources |121
To set the document root for an Apache web server, add a DocumentRoot line to the
httpd.conf configuration file:
DocumentRoot /usr/local/httpd/files
Servers are careful not to let relative URLs back up out of a docroot and expose other
parts of the filesystem. For example, most mature web servers will not permit this
URI to see files above the Joe’s Hardware document root:
http://www.joes-hardware.com/../
Virtually hosted docroots
Virtually hosted web servers host multiple web sites on the same web server, giving
each site its own distinct document root on the server. A virtually hosted web server
identifies the correct document root to use from the IP address or hostname in the
URI or the Host header. This way, two web sites hosted on the same web server can
have completely distinct content, even if the request URIs are identical.
In Figure 5-9, the server hosts two sites: www.joes-hardware.com and www.marys-
antiques.com. The server can distinguish the web sites using the HTTP Host header,
or from distinct IP addresses.
• When request A arrives, the server fetches the file for /docs/joe/index.html.
• When request B arrives, the server fetches the file for /docs/mary/index.html.
Configuring virtually hosted docroots is simple for most web servers. For the popu-
lar Apache web server, you need to configure a VirtualHost block for each virtual
web site, and include the DocumentRoot for each virtual server (Example 5-3).
Figure 5-9. Different docroots for virtually hosted requests
Example 5-3. Apache web server virtual host docroot configuration
<VirtualHost www.joes-hardware.com>
ServerName www.joes-hardware.com
DocumentRoot /docs/joe
/docs/mary/docs/joe
Internet
Request message A
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
Client www.joes-hardware.com
www.marys-antiques.com
GET /index.html HTTP/1.0
Host: www.marys-antiques.com
Request message B
122 |Chapter 5: Web Servers
Look forward to “Virtual Hosting” in Chapter 18 for much more detail about virtual
hosting.
User home directory docroots
Another common use of docroots gives people private web sites on a web server. A
typical convention maps URIs whose paths begin with a slash and tilde (/~) fol-
lowed by a username to a private document root for that user. The private docroot is
often the folder called public_html inside that user’s home directory, but it can be
configured differently (Figure 5-10).
Directory Listings
A web server can receive requests for directory URLs, where the path resolves to a
directory, not a file. Most web servers can be configured to take a few different
actions when a client requests a directory URL:
• Return an error.
• Return a special, default, “index file” instead of the directory.
• Scan the directory, and return an HTML page containing the contents.
Most web servers look for a file named index.html or index.htm inside a directory to
represent that directory. If a user requests a URL for a directory and the directory
TransferLog /logs/joe.access_log
ErrorLog /logs/joe.error_log
</VirtualHost>
<VirtualHost www.marys-antiques.com>
ServerName www.marys-antiques.com
DocumentRoot /docs/mary
TransferLog /logs/mary.access_log
ErrorLog /logs/mary.error_log
</VirtualHost>
...
Figure 5-10. Different docroots for different users
Example 5-3. Apache web server virtual host docroot configuration (continued)
Internet
Request message A
GET /~bob/index.html HTTP/1.0
Client www.joes-hardware.com
www.marys-antiques.com
GET /~betty/index.html HTTP/1.0
Request message B
/home/betty/public_html
/home/bob/public_html
Step 4: Mapping and Accessing Resources |123
contains a file named index.html (or index.htm), the server will return the contents of
that file.
In the Apache web server, you can configure the set of filenames that will be inter-
preted as default directory files using the DirectoryIndex configuration directive. The
DirectoryIndex directive lists all filenames that serve as directory index files, in pre-
ferred order. The following configuration line causes Apache to search a directory for
any of the listed files in response to a directory URL request:
DirectoryIndex index.html index.htm home.html home.htm index.cgi
If no default index file is present when a user requests a directory URI, and if direc-
tory indexes are not disabled, many web servers automatically return an HTML file
listing the files in that directory, and the sizes and modification dates of each file,
including URI links to each file. This file listing can be convenient, but it also allows
nosy people to find files on a web server that they might not normally find.
You can disable the automatic generation of directory index files with the Apache
directive:
Options -Indexes
Dynamic Content Resource Mapping
Web servers also can map URIs to dynamic resources—that is, to programs that gen-
erate content on demand (Figure 5-11). In fact, a whole class of web servers called
application servers connect web servers to sophisticated backend applications. The
web server needs to be able to tell when a resource is a dynamic resource, where the
dynamic content generator program is located, and how to run the program. Most
web servers provide basic mechanisms to identify and map dynamic resources.
Apache lets you map URI pathname components into executable program directo-
ries. When a server receives a request for a URI with an executable path compo-
nent, it attempts to execute a program in a corresponding server directory. For
example, the following Apache configuration directive specifies that all URIs whose
paths begin with /cgi-bin/ should execute corresponding programs found in the
directory /usr/local/etc/httpd/cgi-programs/:
ScriptAlias /cgi-bin/ /usr/local/etc/httpd/cgi-programs/
Apache also lets you mark executable files with a special file extension. This way,
executable scripts can be placed in any directory. The following Apache configura-
tion directive specifies that all web resources ending in .cgi should be executed:
AddHandler cgi-script .cgi
CGI is an early, simple, and popular interface for executing server-side applications.
Modern application servers have more powerful and efficient server-side dynamic
content support, including Microsoft’s Active Server Pages and Java servlets.
124 |Chapter 5: Web Servers
Server-Side Includes (SSI)
Many web servers also provide support for server-side includes. If a resource is
flagged as containing server-side includes, the server processes the resource contents
before sending them to the client.
The contents are scanned for certain special patterns (often contained inside special
HTML comments), which can be variable names or embedded scripts. The special
patterns are replaced with the values of variables or the output of executable scripts.
This is an easy way to create dynamic content.
Access Controls
Web servers also can assign access controls to particular resources. When a request
arrives for an access-controlled resource, the web server can control access based on
the IP address of the client, or it can issue a password challenge to get access to the
resource.
Refer to Chapter 12 for more information about HTTP authentication.
Figure 5-11. A web server can serve static resources as well as dynamic resources
Client Server
Internet
E-commerce
gateway
Real estate search
gateway
Stock trading
gateway
Web cam
gateway
11000101101
Image file
Text file
Filesystem Resources
Step 5: Building Responses |125
Step 5: Building Responses
Once the web server has identified the resource, it performs the action described in
the request method and returns the response message. The response message con-
tains a response status code, response headers, and a response body if one was gener-
ated. HTTP response codes were detailed in “Status Codes” in Chapter 3.
Response Entities
If the transaction generated a response body, the content is sent back with the
response message. If there was a body, the response message usually contains:
• A Content-Type header, describing the MIME type of the response body
• A Content-Length header, describing the size of the response body
• The actual message body content
MIME Typing
The web server is responsible for determining the MIME type of the response body.
There are many ways to configure servers to associate MIME types with resources:
mime.types
The web server can use the extension of the filename to indicate MIME type.
The web server scans a file containing MIME types for each extension to com-
pute the MIME type for each resource. This extension-based type association is
the most common; it is illustrated in Figure 5-12.
Figure 5-12. A web server uses MIME types file to set outgoing Content-Type of resources
Internet
GET /specials/saw-blade.gif HTTP/1.0
Host: www.joes-hardware.com
Client www.joes-hardware.com
HTTP/1.0 200 OK
Content-type: image/gif
Content-length: 8572 Server MIME type table
saw-blade.gif file
application/msword doc
application/postscript ai eps ps
application/powerpoint ppt
audio/mpeg mpga mp2
image/gif gif
image/jpeg jpeg jpg jpe
image/tiff tiff tif
text/html html htm
text/plain txt
video/mpeg mpeg mpg mpe
video/quicktime qt mov
video/x-msvideo avi
x-word/x-vrml wrl vrml
HTTP request message contains
the command and the URI
126 |Chapter 5: Web Servers
Magic typing
The Apache web server can scan the contents of each resource and pattern-
match the content against a table of known patterns (called the magic file) to
determine the MIME type for each file. This can be slow, but it is convenient,
especially if the files are named without standard extensions.
Explicit typing
Web servers can be configured to force particular files or directory contents to
have a MIME type, regardless of the file extension or contents.
Type negotiation
Some web servers can be configured to store a resource in multiple document
formats. In this case, the web server can be configured to determine the “best”
format to use (and the associated MIME type) by a negotiation process with the
user. We’ll discuss this in Chapter 17.
Web servers also can be configured to associate particular files with MIME types.
Redirection
Web servers sometimes return redirection responses instead of success messages. A
web server can redirect the browser to go elsewhere to perform the request. A redirec-
tion response is indicated by a 3XX return code. The Location response header con-
tains a URI for the new or preferred location of the content. Redirects are useful for:
Permanently moved resources
A resource might have been moved to a new location, or otherwise renamed, giv-
ing it a new URL. The web server can tell the client that the resource has been
renamed, and the client can update any bookmarks, etc. before fetching the
resource from its new location. The status code 301 Moved Permanently is used
for this kind of redirect.
Temporarily moved resources
If a resource is temporarily moved or renamed, the server may want to redirect
the client to the new location. But, because the renaming is temporary, the server
wants the client to come back with the old URL in the future and not to update
any bookmarks. The status codes 303 See Other and 307 Temporary Redirect
are used for this kind of redirect.
URL augmentation
Servers often use redirects to rewrite URLs, often to embed context. When the
request arrives, the server generates a new URL containing embedded state infor-
mation and redirects the user to this new URL.*The client follows the redirect,
reissuing the request, but now including the full, state-augmented URL. This is a
* These extended, state-augmented URLs are sometimes called “fat URLs.”
For More Information |127
useful way of maintaining state across transactions. The status codes 303 See
Other and 307 Temporary Redirect are used for this kind of redirect.
Load balancing
If an overloaded server gets a request, the server can redirect the client to a less
heavily loaded server. The status codes 303 See Other and 307 Temporary Redi-
rect are used for this kind of redirect.
Server affinity
Web servers may have local information for certain users; a server can redirect
the client to a server that contains information about the client. The status codes
303 See Other and 307 Temporary Redirect are used for this kind of redirect.
Canonicalizing directory names
When a client requests a URI for a directory name without a trailing slash, most
web servers redirect the client to a URI with the slash added, so that relative
links work correctly.
Step 6: Sending Responses
Web servers face similar issues sending data across connections as they do receiving.
The server may have many connections to many clients, some idle, some sending
data to the server, and some carrying response data back to the clients.
The server needs to keep track of the connection state and handle persistent connec-
tions with special care. For nonpersistent connections, the server is expected to close
its side of the connection when the entire message is sent.
For persistent connections, the connection may stay open, in which case the server
needs to be extra cautious to compute the Content-Length header correctly, or the
client will have no way of knowing when a response ends (see Chapter 4).
Step 7: Logging
Finally, when a transaction is complete, the web server notes an entry into a log file,
describing the transaction performed. Most web servers provide several configurable
forms of logging. Refer to Chapter 21 for more details.
For More Information
For more information on Apache, Jigsaw, and ident, check out:
Apache: The Definitive Guide
Ben Laurie and Peter Laurie, O’Reilly & Associates, Inc.
Professional Apache
Peter Wainwright, Wrox Press.
128 |Chapter 5: Web Servers
http://www.w3c.org/Jigsaw/
Jigsaw—W3C’s Server W3C Consortium Web Site.
http://www.ietf.org/rfc/rfc1413.txt
RFC 1413, “Identification Protocol,” by M. St. Johns.
129
CHAPTER 6
Proxies
Web proxy servers are intermediaries. Proxies sit between clients and servers and act
as “middlemen,” shuffling HTTP messages back and forth between the parties.This
chapter talks all about HTTP proxy servers, the special support for proxy features,
and some of the tricky behaviors you’ll see when you use proxy servers.
In this chapter, we:
• Explain HTTP proxies, contrasting them to web gateways and illustrating how
proxies are deployed.
• Show some of the ways proxies are helpful.
• Describe how proxies are deployed in real networks and how traffic is directed
to proxy servers.
• Show how to configure your browser to use a proxy.
• Demonstrate HTTP proxy requests, how they differ from server requests, and
how proxies can subtly change the behavior of browsers.
• Explain how you can record the path of your messages through chains of proxy
servers, using Via headers and the TRACE method.
• Describe proxy-based HTTP access control.
• Explain how proxies can interoperate between clients and servers, each of which
may support different features and versions.
Web Intermediaries
Web proxy servers are middlemen that fulfill transactions on the client’s behalf.
Without a web proxy, HTTP clients talk directly to HTTP servers. With a web
proxy, the client instead talks to the proxy, which itself communicates with the
server on the client’s behalf. The client still completes the transaction, but through
the good services of the proxy server.
130 |Chapter 6: Proxies
HTTP proxy servers are both web servers and web clients. Because HTTP clients
send request messages to proxies, the proxy server must properly handle the requests
and the connections and return responses, just like a web server. At the same time,
the proxy itself sends requests to servers, so it must also behave like a correct HTTP
client, sending requests and receiving responses (see Figure 6-1). If you are creating
your own HTTP proxy, you’ll need to carefully follow the rules for both HTTP cli-
ents and HTTP servers.
Private and Shared Proxies
A proxy server can be dedicated to a single client or shared among many clients.
Proxies dedicated to a single client are called private proxies. Proxies shared among
numerous clients are called public proxies.
Public proxies
Most proxies are public, shared proxies. It’s more cost effective and easier to
administer a centralized proxy. And some proxy applications, such as caching
proxy servers, become more useful as more users are funneled into the same proxy
server, because they can take advantage of common requests between users.
Private proxies
Dedicated private proxies are not as common, but they do have a place, espe-
cially when run directly on the client computer. Some browser assistant prod-
ucts, as well as some ISP services, run small proxies directly on the user’s PC in
order to extend browser features, improve performance, or host advertising for
free ISP services.
Proxies Versus Gateways
Strictly speaking, proxies connect two or more applications that speak the same pro-
tocol, while gateways hook up two or more parties that speak different protocols. A
gateway acts as a “protocol converter,” allowing a client to complete a transaction
with a server, even when the client and server speak different protocols.
Figure 6-1. A proxy must be both a server and a client
Client Server
Proxy
Request
Proxies act like SERVERS to web clients,
receiving request messages, and
returning response messages
Request
ResponseResponse
Proxies act like CLIENTS to web servers,
sending web request messages, and
receiving web response messages
Why Use Proxies? |131
Figure 6-2 illustrates the difference between proxies and gateways:
• The intermediary device in Figure 6-2a is an HTTP proxy, because the proxy
speaks HTTP to both the client and server.
• The intermediary device in Figure 6-2b is an HTTP/POP gateway, because it ties
an HTTP frontend to a POP email backend. The gateway converts web transac-
tions into the appropriate POP transactions, to allow the user to read email
through HTTP. Web-based email programs such as Yahoo! Mail and MSN Hot-
mail are HTTP email gateways.
In practice, the difference between proxies and gateways is blurry. Because browsers
and servers implement different versions of HTTP, proxies often do some amount of
protocol conversion. And commercial proxy servers implement gateway functional-
ity to support SSL security protocols, SOCKS firewalls, FTP access, and web-based
applications. We’ll talk more about gateways in Chapter 8.
Why Use Proxies?
Proxy servers can do all kinds of nifty and useful things. They can improve security,
enhance performance, and save money. And because proxy servers can see and touch
all the passing HTTP traffic, proxies can monitor and modify the traffic to imple-
ment many useful value-added web services. Here are examples of just a few of the
ways proxies can be used:
Child filter (Figure 6-3)
Elementary schools use filtering proxies to block access to adult content, while
providing unhindered access to educational sites. As shown in Figure 6-3, the
Figure 6-2. Proxies speak the same protocol; gateways tie together different protocols
Browser Web server
Web proxy
HTTP HTTP
(a) HTTP/HTTP proxy
Browser Email serverWeb/email
gateway
HTTP POP
(b) HTTP/POP gateway
132 |Chapter 6: Proxies
proxy might permit unrestricted access to educational content but forcibly deny
access to sites that are inappropriate for children.*
Document access controller (Figure 6-4)
Proxy servers can be used to implement a uniform access-control strategy across
a large set of web servers and web resources and to create an audit trail. This is
useful in large corporate settings or other distributed bureaucracies.
All the access controls can be configured on the centralized proxy server, with-
out requiring the access controls to be updated frequently on numerous web
servers, of different makes and models, administered by different organizations.†
In Figure 6-4, the centralized access-control proxy:
• Permits client 1 to access news pages from server A without restriction
• Gives client 2 unrestricted access to Internet content
• Requires a password from client 3 before allowing access to server B
Security firewall (Figure 6-5)
Network security engineers often use proxy servers to enhance security. Proxy
servers restrict which application-level protocols flow in and out of an organiza-
tion, at a single secure point in the network. They also can provide hooks to
scrutinize that traffic (Figure 6-5), as used by virus-eliminating web and email
proxies.
Figure 6-3. Proxy application example: child-safe Internet filter
* Several companies and nonprofit organizations provide filtering software and maintain “blacklists” in order
to identify and restrict access to objectionable content.
† To prevent sophisticated users from willfully bypassing the control proxy, the web servers can be statically
configured to accept requests only from the proxy servers.
Server
Server
Child user
Child user School’s filtering
proxy
OK
Internet
DENY
Why Use Proxies? |133
Web cache (Figure 6-6)
Proxy caches maintain local copies of popular documents and serve them on
demand, reducing slow and costly Internet communication.
In Figure 6-6, clients 1 and 2 access object A from a nearby web cache, while cli-
ents 3 and 4 access the document from the origin server.
Figure 6-4. Proxy application example: centralized document access control
Figure 6-5. Proxy application example: security firewall
Server B
General
news
Client 1
Client 2
Client 3
To the Internet
Secret
financial
data
What is the password for
the financial data?
Intended request
to server B
blocked
Access
control
proxy Server A
General
news
Local area
network
Internet
Server
Client
Client
Client
Internet
Server
Server
Filtering router
Firewall
proxy
Filtering router
Virus
Firewall Firewall
134 |Chapter 6: Proxies
Surrogate (Figure 6-7)
Proxies can masquerade as web servers. These so-called surrogates or reverse
proxies receive real web server requests, but, unlike web servers, they may initiate
communication with other servers to locate the requested content on demand.
Surrogates may be used to improve the performance of slow web servers for com-
mon content. In this configuration, the surrogates often are called server accelera-
tors (Figure 6-7). Surrogates also can be used in conjunction with content-routing
functionality to create distributed networks of on-demand replicated content.
Content router (Figure 6-8)
Proxy servers can act as “content routers,” vectoring requests to particular web
servers based on Internet traffic conditions and type of content.
Content routers also can be used to implement various service-level offerings.
For example, content routers can forward requests to nearby replica caches if the
Figure 6-6. Proxy application example: web cache
Figure 6-7. Proxy application example: surrogate (in a server accelerator deployment)
Origin
server
Client 1
Client 2
Client 3
Client 4
Web caching
proxy
Client Server
Internet
Surrogate
(also known as a reverse proxy
or a server accelerator)
Why Use Proxies? |135
user or content provider has paid for higher performance (Figure 6-8), or route
HTTP requests through filtering proxies if the user has signed up for a filtering
service. Many interesting services can be constructed using adaptive content-
routing proxies.
Transcoder (Figure 6-9)
Proxy servers can modify the body format of content before delivering it to clients.
This transparent translation between data representations is called transcoding.*
Transcoding proxies can convert GIF images into JPEG images as they fly by, to
reduce size. Images also can be shrunk and reduced in color intensity to be view-
able on television sets. Likewise, text files can be compressed, and small text
summaries of web pages can be generated for Internet-enabled pagers and smart
phones. It’s even possible for proxies to convert documents into foreign lan-
guages on the fly!
Figure 6-9 shows a transcoding proxy that converts English text into Spanish
text and also reformats HTML pages into simpler text that can displayed on the
small screen of a mobile phone.
Figure 6-8. Proxy application example: content routing
* Some people distinguish “transcoding” and “translation,” defining transcoding as relatively simple conver-
sions of the encoding of the data (e.g., lossless compression) and translation as more significant reformatting
or semantic changes of the data. We use the term transcoding to mean any intermediary-based modification
of the content.
Server A
Sharon
Rob
Luis
Server B
Content
router
Content
router
Server A paid to have content distributed
to replica caches, but server B did not.
The content router steers Luis to a replica cache for
A’s pages but to the origin server for B’s pages.
Sharon paid for the performance, so the content
router sends her to the nearby cache. Rob didn’t,
so the content router sends him to the origin server.
R1
R2
Web caching
proxy
136 |Chapter 6: Proxies
Anonymizer (Figure 6-10)
Anonymizer proxies provide heightened privacy and anonymity, by actively
removing identifying characteristics from HTTP messages (e.g., client IP address,
From header, Referer header, cookies, URI session IDs).*
In Figure 6-10, the anonymizing proxy makes the following changes to the user’s
messages to increase privacy:
• The user’s computer and OS type is removed from the User-Agent header.
• The From header is removed to protect the user’s email address.
• The Referer header is removed to obscure other sites the user has visited.
• The Cookie headers are removed to eliminate profiling and identity data.
Figure 6-9. Proxy application example: content transcoder
* However, because identifying information is removed, the quality of the user’s browsing experience may be
diminished, and some web sites may not function properly.
Figure 6-10. Proxy application example: anonymizer
Blanco
Negro
Naranja amanecer
Spanish-
speaking
client
Web-enabled
mobile phone
Summer Beach Shirts
You’ll get lots of smiles and
winks when you wear our
summer beach shirts.
1) White
2) Black
3) Sunrise orange
Playeras de Verano
Obtendra muchas sonrisas
y guiñios cuando use nuestras
playeras de verano.
Transcoding
proxy
Origin
server
Summer Beach Shirts
You’ll get lots of smiles and
winks when you wear our
summer beach shirts.
White
Black
Sunrise orange
Client Server
GET /something/file.html HTTP/1.0
Date: Sun, 01 Oct 2000 23:25:17 GMT
User-agent: Mozilla/4.75 (Win98; U)
From: joe@joes-hardware.com
Referer: http://www.irs.gov/tax-audits.html
Cookie: profile="football,lite beer"
Cookie: income-bracket="30K-45K"
Anonymizing
proxy
GET /something/file.html HTTP/1.0
Date: Sun, 01 Oct 2000 23:25:17 GMT
User-agent: Mozilla/4.75
Anonymized message doesn’t contain the
common identifying information headers
Where Do Proxies Go? |137
Where Do Proxies Go?
The previous section explained what proxies do. Now let’s talk about where proxies
sit when they are deployed into a network architecture. We’ll cover:
• How proxies can be deployed into networks
• How proxies can chain together into hierarchies
• How traffic gets directed to a proxy server in the first place
Proxy Server Deployment
You can place proxies in all kinds of places, depending on their intended uses.
Figure 6-11 sketches a few ways proxy servers can be deployed.
Egress proxy (Figure 6-11a)
You can stick proxies at the exit points of local networks to control the traffic
flow between the local network and the greater Internet. You might use egress
proxies in a corporation to offer firewall protection against malicious hackers
outside the enterprise or to reduce bandwidth charges and improve perfor-
mance of Internet traffic. An elementary school might use a filtering egress proxy
to prevent precocious students from browsing inappropriate content.
Access (ingress) proxy (Figure 6-11b)
Proxies are often placed at ISP access points, processing the aggregate requests
from the customers. ISPs use caching proxies to store copies of popular docu-
ments, to improve the download speed for their users (especially those with
high-speed connections) and reduce Internet bandwidth costs.
Surrogates (Figure 6-11c)
Proxies frequently are deployed as surrogates (also commonly called reverse
proxies) at the edge of the network, in front of web servers, where they can field
all of the requests directed at the web server and ask the web server for resources
only when necessary. Surrogates can add security features to web servers or
improve performance by placing fast web server caches in front of slower web
servers. Surrogates typically assume the name and IP address of the web server
directly, so all requests go to the proxy instead of the server.
Network exchange proxy (Figure 6-11d)
With sufficient horsepower, proxies can be placed in the Internet peering
exchange points between networks, to alleviate congestion at Internet junctions
through caching and to monitor traffic flows.*
* Core proxies often are deployed where Internet bandwidth is very expensive (especially in Europe). Some
countries (such as the UK) also are evaluating controversial proxy deployments to monitor Internet traffic
for national security concerns.
138 |Chapter 6: Proxies
Proxy Hierarchies
Proxies can be cascaded in chains called proxy hierarchies. In a proxy hierarchy, mes-
sages are passed from proxy to proxy until they eventually reach the origin server
(and then are passed back through the proxies to the client), as shown in Figure 6-12.
Proxy servers in a proxy hierarchy are assigned parent and child relationships. The
next inbound proxy (closer to the server) is called the parent, and the next outbound
proxy (closer to the client) is called the child. In Figure 6-12, proxy 1 is the child
Figure 6-11. Proxies can be deployed many ways, depending on their intended use
Client
Client Server
Proxy
Internet
(a) Private LAN egress proxy
Client
Client Server
Internet
(b) ISP access proxy
(c) Surrogate
Client Server
(d) Network exchange proxy
Local
network
Proxy
Client
Client Server
Proxy
Internet Local
network
Network 1 Network 2
Proxy
Router Router
Where Do Proxies Go? |139
proxy of proxy 2. Likewise, proxy 2 is the child proxy of proxy 3, and proxy 3 is the
parent proxy of proxy 2.
Proxy hierarchy content routing
The proxy hierarchy in Figure 6-12 is static—proxy 1 always forwards messages to
proxy 2, and proxy 2 always forwards messages to proxy 3. However, hierarchies do
not have to be static. A proxy server can forward messages to a varied and changing
set of proxy servers and origin servers, based on many factors.
For example, in Figure 6-13, the access proxy routes to parent proxies or origin serv-
ers in different circumstances:
• If the requested object belongs to a web server that has paid for content distribu-
tion, the proxy could route the request to a nearby cache server that would
either return the cached object or fetch it if it wasn’t available.
• If the request was for a particular type of image, the access proxy might route the
request to a dedicated compression proxy that would fetch the image and then
compress it, so it would download faster across a slow modem to the client.
Figure 6-12. Three-level proxy hierarchy
Figure 6-13. Proxy hierarchies can be dynamic, changing for each request
Client Proxy 1
(child of proxy 2) Origin server
Proxy 2
(child of proxy 3
parent of proxy 1)
Proxy 3
(parent of proxy 2)
Client Access proxy
Internet
Web servers around
the globe
Dedicated cache server for
specially-subscribed objects
Compressor
proxy
Caching proxy
140 |Chapter 6: Proxies
Here are a few other examples of dynamic parent selection:
Load balancing
A child proxy might pick a parent proxy based on the current level of workload
on the parents, to spread the load around.
Geographic proximity routing
A child proxy might select a parent responsible for the origin server’s geographic
region.
Protocol/type routing
A child proxy might route to different parents and origin servers based on the
URI. Certain types of URIs might cause the requests to be transported through
special proxy servers, for special protocol handling.
Subscription-based routing
If publishers have paid extra money for high-performance service, their URIs
might be routed to large caches or compression engines to improve performance.
Dynamic parenting routing logic is implemented differently in different products,
including configuration files, scripting languages, and dynamic executable plug-ins.
How Proxies Get Traffic
Because clients normally talk directly to web servers, we need to explain how HTTP
traffic finds its way to a proxy in the first place. There are four common ways to
cause client traffic to get to a proxy:
Modify the client
Many web clients, including Netscape and Microsoft browsers, support both
manual and automated proxy configuration. If a client is configured to use a
proxy server, the client sends HTTP requests directly and intentionally to the
proxy, instead of to the origin server (Figure 6-14a).
Modify the network
There are several techniques where the network infrastructure intercepts and
steers web traffic into a proxy, without the client’s knowledge or participation.
This interception typically relies on switching and routing devices that watch for
HTTP traffic, intercept it, and shunt the traffic into a proxy, without the client’s
knowledge (Figure 6-14b). This is called an intercepting proxy.*
Modify the DNS namespace
Surrogates, which are proxy servers placed in front of web servers, assume the
name and IP address of the web server directly, so all requests go to them instead
* Intercepting proxies commonly are called “transparent proxies,” because you connect to them without being
aware of their presence. Because the term “transparency” already is used in the HTTP specifications to indi-
cate functions that don’t change semantic behavior, the standards community suggests using the term “inter-
ception” for traffic capture. We adopt this nomenclature here.
Client Proxy Settings |141
of to the server (Figure 6-14c). This can be arranged by manually editing the
DNS naming tables or by using special dynamic DNS servers that compute the
appropriate proxy or server to use on-demand. In some installations, the IP
address and name of the real server is changed and the surrogate is given the
former address and name.
Modify the web server
Some web servers also can be configured to redirect client requests to a proxy by
sending an HTTP redirection command (response code 305) back to the client.
Upon receiving the redirect, the client transacts with the proxy (Figure 6-14d).
The next section explains how to configure clients to send traffic to proxies.
Chapter 20 will explain how to configure the network, DNS, and servers to redirect
traffic to proxy servers.
Client Proxy Settings
All modern web browsers let you configure the use of proxies. In fact, many brows-
ers provide multiple ways of configuring proxies, including:
Manual configuration
You explicitly set a proxy to use.
Browser preconfiguration
The browser vendor or distributor manually preconfigures the proxy setting of
the browser (or any other web client) before delivering it to customers.
Figure 6-14. There are many techniques to direct web requests to proxies
Client Server
Proxy
(a) Client configured to use proxy
Client Server
(b) Network intercepts and redirects traffic to proxy
Client Server
Proxy
(assuming the
web server’s
name)
(c) Surrogate stands in for web server
Client Server
(d) Server redirects HTTP requests to proxy
Router
Proxy
Proxy
142 |Chapter 6: Proxies
Proxy auto-configuration (PAC)
You provide a URI to a JavaScript proxy auto-configuration (PAC) file; the client
fetches the JavaScript file and runs it to decide if it should use a proxy and, if so,
which proxy server to use.
WPAD proxy discovery
Some browsers support the Web Proxy Autodiscovery Protocol (WPAD), which
automatically detects a “configuration server” from which the browser can
download an auto-configuration file.*
Client Proxy Configuration: Manual
Many web clients allow you to configure proxies manually. Both Netscape Navigator
and Microsoft Internet Explorer have convenient support for proxy configuration.
In Netscape Navigator 6, you specify proxies through the menu selection Edit ➝Pref-
erences ➝Advanced ➝Proxies and then selecting the “Manual proxy configuration”
radio button.
In Microsoft Internet Explorer 5, you can manually specify proxies from the Tools ➝
Internet Options menu, by selecting a connection, pressing “Settings,” checking the
“Use a proxy server” box, and clicking “Advanced.”
Other browsers have different ways of making manual configuration changes, but
the idea is the same: specifying the host and port for the proxy. Several ISPs ship cus-
tomers preconfigured browsers, or customized operating systems, that redirect web
traffic to proxy servers.
Client Proxy Configuration: PAC Files
Manual proxy configuration is simple but inflexible. You can specify only one proxy
server for all content, and there is no support for failover. Manual proxy configura-
tion also leads to administrative problems for large organizations. With a large base
of configured browsers, it’s difficult or impossible to reconfigure every browser if you
need to make changes.
Proxy auto-configuration (PAC) files are a more dynamic solution for proxy configu-
ration, because they are small JavaScript programs that compute proxy settings on
the fly. Each time a document is accessed, a JavaScript function selects the proper
proxy server.
To use PAC files, configure your browser with the URI of the JavaScript PAC file
(configuration is similar to manual configuration, but you provide a URI in an “auto-
matic configuration” box). The browser will fetch the PAC file from this URI and use
* Currently supported only by Internet Explorer.
Client Proxy Settings |143
the JavaScript logic to compute the proper proxy server for each access. PAC files
typically have a .pac suffix and the MIME type “application/x-ns-proxy-autoconfig.”
Each PAC file must define a function called FindProxyForURL(url,host) that com-
putes the proper proxy server to use for accessing the URI. The return value of the
function can be any of the values in Table 6-1.
The PAC file in Example 6-1 mandates one proxy for HTTP transactions, another
proxy for FTP transactions, and direct connections for all other kinds of transactions.
For more details about PAC files, refer to Chapter 20.
Client Proxy Configuration: WPAD
Another mechanism for browser configuration is the Web Proxy Autodiscovery Pro-
tocol (WPAD). WPAD is an algorithm that uses an escalating strategy of discovery
mechanisms to find the appropriate PAC file for the browser automatically. A client
that implements the WPAD protocol will:
• Use WPAD to find the PAC URI.
• Fetch the PAC file given the URI.
• Execute the PAC file to determine the proxy server.
• Use the proxy server for requests.
WPAD uses a series of resource-discovery techniques to determine the proper PAC
file. Multiple discovery techniques are used, because not all organizations can use all
techniques. WPAD attempts each technique, one by one, until it succeeds.
Table 6-1. Proxy auto-configuration script return values
FindProxyForURL return value Description
DIRECT Connections should be made directly, without any proxies.
PROXY host:port The specified proxy should be used.
SOCKS host:port The specified SOCKS server should be used.
Example 6-1. Example proxy auto-configuration file
function FindProxyForURL(url, host) {
if (url.substring(0,5) == "http:") {
return "PROXY http-proxy.mydomain.com:8080";
} else if (url.substring(0,4) =="ftp:") {
return "PROXY ftp-proxy.mydomain.com:8080";
} else {
return "DIRECT";
}
}
144 |Chapter 6: Proxies
The current WPAD specification defines the following techniques, in order:
• Dynamic Host Discovery Protocol (DHCP)
• Service Location Protocol (SLP)
• DNS well-known hostnames
• DNS SRV records
• DNS service URIs in TXT records
For more information, consult Chapter 20.
Tricky Things About Proxy Requests
This section explains some of the tricky and much misunderstood aspects of proxy
server requests, including:
• How the URIs in proxy requests differ from server requests
• How intercepting and reverse proxies can obscure server host information
• The rules for URI modification
• How proxies impact a browser’s clever URI auto-completion or hostname-
expansion features
Proxy URIs Differ from Server URIs
Web server and web proxy messages have the same syntax, with one exception. The
URI in an HTTP request message differs when a client sends the request to a server
instead of a proxy.
When a client sends a request to a web server, the request line contains only a par-
tial URI (without a scheme, host, or port), as shown in the following example:
GET /index.html HTTP/1.0
User-Agent: SuperBrowserv1.3
When a client sends a request to a proxy, however, the request line contains the full
URI. For example:
GET http://www.marys-antiques.com/index.html HTTP/1.0
User-Agent: SuperBrowser v1.3
Why have two different request formats, one for proxies and one for servers? In the
original HTTP design, clients talked directly to a single server. Virtual hosting did
not exist, and no provision was made for proxies. Because a single server knows its
own hostname and port, to avoid sending redundant information, clients sent just
the partial URI, without the scheme and host (and port).
When proxies emerged, the partial URIs became a problem. Proxies needed to know
the name of the destination server, so they could establish their own connections to
Tricky Things About Proxy Requests |145
the server. And proxy-based gateways needed the scheme of the URI to connect to
FTP resources and other schemes. HTTP/1.0 solved the problem by requiring the full
URI for proxy requests, but it retained partial URIs for server requests (there were
too many servers already deployed to change all of them to support full URIs).*
So we need to send partial URIs to servers, and full URIs to proxies. In the case of
explicitly configured client proxy settings, the client knows what type of request to
issue:
• When the client is not set to use a proxy, it sends the partial URI (Figure 6-15a).
• When the client is set to use a proxy, it sends the full URI (Figure 6-15b).
* HTTP/1.1 now requires servers to handle full URIs for both proxy and server requests, but in practice, many
deployed servers still accept only partial URIs.
Figure 6-15. Intercepting proxies will get server requests
Client Origin server
(a) Server request GET /index.html HTTP/1.0
User-agent: SuperBrowser v1.3
Client Origin server
(b) Explicit proxy request GET http://www.marys-antiques.com/index.html HTTP/1.0
User-agent: SuperBrowser v1.3
Client
(c) Surrogate (reverse proxy) request GET /index.html HTTP/1.0
User-agent: SuperBrowser v1.3
Client Origin server
(d) Intercepting proxy request
GET /index.html HTTP/1.0
User-agent: SuperBrowser v1.3
Surrogate
Intercepting proxy
(Proxy explicitly configured) Proxy server
Origin server
(Server hostname points to the surrogate proxy)
146 |Chapter 6: Proxies
The Same Problem with Virtual Hosting
The proxy “missing scheme/host/port” problem is the same problem faced by
virtually hosted web servers. Virtually hosted web servers share the same physi-
cal web server among many web sites. When a request comes in for the partial
URI /index.html, the virtually hosted web server needs to know the hostname of
the intended web site (see “Virtually hosted docroots” in Chapter 5 and “Virtual
Hosting” in Chapter 18 for more information).
In spite of the problems being similar, they were solved in different ways:
• Explicit proxies solve the problem by requiring a full URI in the request message.
• Virtually hosted web servers require a Host header to carry the host and port
information.
Intercepting Proxies Get Partial URIs
As long as the clients properly implement HTTP, they will send full URIs in requests
to explicitly configured proxies. That solves part of the problem, but there’s a catch:
a client will not always know it’s talking to a proxy, because some proxies may be
invisible to the client. Even if the client is not configured to use a proxy, the client’s
traffic still may go through a surrogate or intercepting proxy. In both of these cases,
the client will think it’s talking to a web server and won’t send the full URI:
•Asurrogate, as described earlier, is a proxy server taking the place of the origin
server, usually by assuming its hostname or IP address. It receives the web server
request and may serve cached responses or proxy requests to the real server. A
client cannot distinguish a surrogate from a web server, so it sends partial URIs
(Figure 6-15c).
•Anintercepting proxy is a proxy server in the network flow that hijacks traffic
from the client to the server and either serves a cached response or proxies it.
Because the intercepting proxy hijacks client-to-server traffic, it will receive par-
tial URIs that are sent to web servers (Figure 6-15d).*
Proxies Can Handle Both Proxy and Server Requests
Because of the different ways that traffic can be redirected into proxy servers,
general-purpose proxy servers should support both full URIs and partial URIs in
request messages. The proxy should use the full URI if it is an explicit proxy request
or use the partial URI and the virtual Host header if it is a web server request.
* Intercepting proxies also might intercept client-to-proxy traffic in some circumstances, in which case the
intercepting proxy might get full URIs and need to handle them. This doesn’t happen often, because explicit
proxies normally communicate on a port different from that used by HTTP (usually 8080 instead of 80), and
intercepting proxies usually intercept only port 80.
Tricky Things About Proxy Requests |147
The rules for using full and partial URIs are:
• If a full URI is provided, the proxy should use it.
• If a partial URI is provided, and a Host header is present, the Host header
should be used to determine the origin server name and port number.
• If a partial URI is provided, and there is no Host header, the origin server needs
to be determined in some other way:
— If the proxy is a surrogate, standing in for an origin server, the proxy can be
configured with the real server’s address and port number.
— If the traffic was intercepted, and the interceptor makes the original IP
address and port available, the proxy can use the IP address and port num-
ber from the interception technology (see Chapter 20).
— If all else fails, the proxy doesn’t have enough information to determine the
origin server and must return an error message (often suggesting that the
user upgrade to a modern browser that supports Host headers).*
In-Flight URI Modification
Proxy servers need to be very careful about changing the request URI as they for-
ward messages. Slight changes in the URI, even if they seem benign, may create
interoperability problems with downstream servers.
In particular, some proxies have been known to “canonicalize” URIs into a standard
form before forwarding them to the next hop. Seemingly benign transformations,
such as replacing default HTTP ports with an explicit “:80”, or correcting URIs by
replacing illegal reserved characters with their properly escaped substitutions, can
cause interoperation problems.
In general, proxy servers should strive to be as tolerant as possible. They should not
aim to be “protocol policemen” looking to enforce strict protocol compliance,
because this could involve significant disruption of previously functional services.
In particular, the HTTP specifications forbid general intercepting proxies from
rewriting the absolute path parts of URIs when forwarding them. The only excep-
tion is that they can replace an empty path with “/”.
URI Client Auto-Expansion and Hostname Resolution
Browsers resolve request URIs differently, depending on whether or not a proxy is
present. Without a proxy, the browser takes the URI you type in and tries to find a
corresponding IP address. If the hostname is found, the browser tries the corre-
sponding IP addresses until it gets a successful connection.
* This shouldn’t be done casually. Users will receive cryptic error pages they never got before.
148 |Chapter 6: Proxies
But if the host isn’t found, many browsers attempt to provide some automatic
“expansion” of hostnames, in case you typed in a “shorthand” abbreviation of the
host (refer back to “Expandomatic URLs” in Chapter 2):*
• Many browsers attempt adding a “www.” prefix and a “.com” suffix, in case you
just entered the middle piece of a common web site name (e.g., to let people
enter “yahoo” instead of “www.yahoo.com”).
• Some browsers even pass your unresolvable URI to a third-party site, which
attempts to correct spelling mistakes and suggest URIs you may have intended.
• In addition, the DNS configuration on most systems allows you to enter just the
prefix of the hostname, and the DNS automatically searches the domain. If you are
in the domain “oreilly.com” and type in the hostname “host7,” the DNS automati-
cally attempts to match “host7.oreilly.com”. It’s not a complete, valid hostname.
URI Resolution Without a Proxy
Figure 6-16 shows an example of browser hostname auto-expansion without a
proxy. In steps 2a–3c, the browser looks up variations of the hostname until a valid
hostname is found.
Here’s what’s going on in this figure:
• In Step 1, the user types “oreilly” into the browser’s URI window. The browser
uses “oreilly” as the hostname and assumes a default scheme of “http://”, a
default port of “80”, and a default path of “/”.
• In Step 2a, the browser looks up host “oreilly.” This fails.
* Most browsers let you type in “yahoo” and auto-expand that into “www.yahoo.com.” Similarly, browsers
let you omit the “http://” prefix and insert it if it’s missing.
Figure 6-16. Browser auto-expands partial hostnames when no explicit proxy is present
Client
(1) User types “oreilly” into
browser’s URI location window
(3a) The browser does auto-expansion,
converting “oreilly” into “www.oreilly.com”
DNS server
(2b) Failed, host unknown
(2a) Browser looks up host “oreilly” via DNS
(3b) Browser looks up host “www.oreilly.com” via DNS
(3c) Success! Get IP addresses back
www.oreilly.com
(4a) Browser tries to connect to IP addresses, one by one, until connect successful
(4b) Success; connection established
(5a) Browser sends HTTP request
(5b) Browser gets HTTP response
Tricky Things About Proxy Requests |149
• In Step 3a, the browser auto-expands the hostname and asks the DNS to resolve
“www.oreilly.com.” This is successful.
• The browser then successfully connects to www.oreilly.com.
URI Resolution with an Explicit Proxy
When you use an explicit proxy the browser no longer performs any of these conve-
nience expansions, because the user’s URI is passed directly to the proxy.
As shown in Figure 6-17, the browser does not auto-expand the partial hostname
when there is an explicit proxy. As a result, when the user types “oreilly” into the
browser’s location window, the proxy is sent “http://oreilly/” (the browser adds the
default scheme and path but leaves the hostname as entered).
For this reason, some proxies attempt to mimic as much as possible of the browser’s
convenience services as they can, including “www...com” auto-expansion and addi-
tion of local domain suffixes.*
URI Resolution with an Intercepting Proxy
Hostname resolution is a little different with an invisible intercepting proxy, because
as far as the client is concerned, there is no proxy! The behavior proceeds much like
the server case, with the browser auto-expanding hostnames until DNS success. But
a significant difference occurs when the connection to the server is made, as
Figure 6-18 illustrates.
Figure 6-17. Browser does not auto-expand partial hostnames when there is an explicit proxy
* But, for widely shared proxies, it may be impossible to know the proper domain suffix for individual users.
Client
(1) User types “oreilly” into
browser’s URI location window
(3a) The browser does auto-expansion,
converting “oreilly” into “www.oreilly.com”
DNS server
(2a) Proxy is explicitly configured,
so the browser looks up the address
of the proxy server using DNS
(2b) Success! Get proxy server
IP addresses
www.oreilly.com
(3a) Browser tries to connect to proxy
(3b) Success; connection established
(4a) Browser sends HTTP request Proxy
GET http://oreilly/ HTTP/1.0
Proxy-connection: Keep-Alive
User-agent: Mozilla/4.72[en] (Win98:I)
Host: oreilly
Accept: */*
Accept-encoding: gzip
Accept-language: en
Accept-charset: iso-8859-1,*,utf-8
Request message, as sent in (4a)
(4b) Proxy gets a partial hostname
in the request, because the client
did not auto-expand it.
150 |Chapter 6: Proxies
Figure 6-18 demonstrates the following transaction:
• In Step 1, the user types “oreilly” into the browser’s URI location window.
• In Step 2a, the browser looks up the host “oreilly” via DNS, but the DNS server
fails and responds that the host is unknown, as shown in Step 2b.
• In Step 3a, the browser does auto-expansion, converting “oreilly” into “www.
oreilly.com.” In Step 3b, the browser looks up the host “www.oreilly.com” via
DNS. This time, as shown in Step 3c, the DNS server is successful and returns IP
addresses back to the browser.
• In Step 4a, the client already has successfully resolved the hostname and has a
list of IP addresses. Normally, the client tries to connect to each IP address until
it succeeds, because some of the IP addresses may be dead. But with an inter-
cepting proxy, the first connection attempt is terminated by the proxy server, not
the origin server. The client believes it is successfully talking to the web server,
but the web server might not even be alive.
• When the proxy finally is ready to interact with the real origin server (Step 5b),
the proxy may find that the IP address actually points to a down server. To pro-
vide the same level of fault tolerance provided by the browser, the proxy needs
to try other IP addresses, either by reresolving the hostname in the Host header
or by doing a reverse DNS lookup on the IP address. It is important that both
intercepting and explicit proxy implementations support fault tolerance on DNS
resolution to dead servers, because when browsers are configured to use an
explicit proxy, they rely on the proxy for fault tolerance.
Tracing Messages
Today, it’s not uncommon for web requests to go through a chain of two or more
proxies on their way from the client to the server (Figure 6-19). For example, many
Figure 6-18. Browser doesn’t detect dead server IP addresses when using intercepting proxies
Client
(1)
(3a)
DNS server
(2b)
(2a)
(3b)
(3c)
www.oreilly.com
(4a)
(4b)
(5a)
Interceptor
Proxy
(5b)
Tracing Messages |151
corporations use caching proxy servers to access the Internet, for security and cost
savings, and many large ISPs use proxy caches to improve performance and imple-
ment features. A significant percentage of web requests today go through proxies. At
the same time, it’s becoming increasingly popular to replicate content on banks of
surrogate caches scattered around the globe, for performance reasons.
Proxies are developed by different vendors. They have different features and bugs
and are administrated by various organizations.
As proxies become more prevalent, you need to be able to trace the flow of messages
across proxies and to detect any problems, just as it is important to trace the flow of
IP packets across different switches and routers.
The Via Header
The Via header field lists information about each intermediate node (proxy or gate-
way) through which a message passes. Each time a message goes through another
node, the intermediate node must be added to the end of the Via list.
The following Via string tells us that the message traveled through two proxies. It
indicates that the first proxy implemented the HTTP/1.1 protocol and was called
proxy-62.irenes-isp.net, and that the second proxy implemented HTTP/1.0 and was
called cache.joes-hardware.com:
Via: 1.1 proxy-62.irenes-isp.net, 1.0 cache.joes-hardware.com
The Via header field is used to track the forwarding of messages, diagnose message
loops, and identify the protocol capabilities of all senders along the request/response
chain (Figure 6-20).
Proxies also can use Via headers to detect routing loops in the network. A proxy
should insert a unique string associated with itself in the Via header before sending
out a request and should check for the presence of this string in incoming requests to
detect routing loops in the network.
Figure 6-19. Access proxies and CDN proxies create two-level proxy hierarchies
Client ISP proxy Internet
Surrogate cache bank
Web server
152 |Chapter 6: Proxies
Via syntax
The Via header field contains a comma-separated list of waypoints. Each waypoint
represents an individual proxy server or gateway hop and contains information about
the protocol and address of that intermediate node. Here is an example of a Via
header with two waypoints:
Via = 1.1 cache.joes-hardware.com, 1.1 proxy.irenes-isp.net
The formal syntax for a Via header is shown here:
Via = "Via" ":" 1#( waypoint )
waypoint = ( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
received-by = ( host [ ":" port ] ) | pseudonym
Note that each Via waypoint contains up to four components: an optional protocol
name (defaults to HTTP), a required protocol version, a required node name, and an
optional descriptive comment:
Protocol name
The protocol received by an intermediary. The protocol name is optional if the
protocol is HTTP. Otherwise, the protocol name is prepended to the version,
separated by a “/”. Non-HTTP protocols can occur when gateways connect
HTTP requests for other protocols (HTTPS, FTP, etc.).
Protocol version
The version of the message received. The format of the version depends on the
protocol. For HTTP, the standard version numbers are used (“1.0”, “1.1”, etc.).
The version is included in the Via field, so later applications will know the proto-
col capabilities of all previous intermediaries.
Node name
The host and optional port number of the intermediary (if the port isn’t
included, you can assume the default port for the protocol). In some cases an
organization might not want to give out the real hostname, for privacy reasons,
in which case it may be replaced by a pseudonym.
Figure 6-20. Via header example
Client
proxy-62.irenes-isp.net
(HTTP/1.1) www.joes-hardware.com
cache.joes-hardware.com
(HTTP/1.0)
GET /index.html HTTP/1.0
Accept: text/html
Host: www.joes-hardware.com
Via: 1.1 proxy-62.irenes-isp.net, 1.0 cache.joes-hardware.com
Request message (as received by server)
Tracing Messages |153
Node comment
An optional comment that further describes the intermediary node. It’s com-
mon to include vendor and version information here, and some proxy servers
also use the comment field to include diagnostic information about the events
that occurred on that device.*
Via request and response paths
Both request and response messages pass through proxies, so both request and
response messages have Via headers.
Because requests and responses usually travel over the same TCP connection,
response messages travel backward across the same path as the requests. If a request
message goes through proxies A, B, and C, the corresponding response message trav-
els through proxies C, B, then A. So, the Via header for responses is almost always
the reverse of the Via header for responses (Figure 6-21).
Via and gateways
Some proxies provide gateway functionality to servers that speak non-HTTP proto-
cols. The Via header records these protocol conversions, so HTTP applications can
be aware of protocol capabilities and conversions along the proxy chain. Figure 6-22
shows an HTTP client requesting an FTP URI through an HTTP/FTP gateway.
The client sends an HTTP request for ftp://http-guide.com/pub/welcome.txt to the
gateway proxy.irenes-isp.net. The proxy, acting as a protocol gateway, retrieves the
desired object from the FTP server, using the FTP protocol. The proxy then sends
the object back to the client in an HTTP response, with this Via header field:
Via: FTP/1.0 proxy.irenes-isp.net (Traffic-Server/5.0.1-17882 [cMs f ])
* For example, caching proxy servers may include hit/miss information.
Figure 6-21. The response Via is usually the reverse of the request Via
Client Server
ABC
Request Via header
via: 1.1 A, 1.1 B, 1.1 C
Reponse Via header
via: 1.1 C, 1.1 B, 1.1 A
154 |Chapter 6: Proxies
Notice the received protocol is FTP. The optional comment contains the brand and
version number of the proxy server and some vendor diagnostic information. You
can read all about gateways in Chapter 8.
The Server and Via headers
The Server response header field describes the software used by the origin server.
Here are a few examples:
Server: Apache/1.3.14 (Unix) PHP/4.0.4
Server: Netscape-Enterprise/4.1
Server: Microsoft-IIS/5.0
If a response message is being forwarded through a proxy, make sure the proxy does
not modify the Server header. The Server header is meant for the origin server.
Instead, the proxy should add a Via entry.
Privacy and security implications of Via
There are some cases when we want don’t want exact hostnames in the Via string. In
general, unless this behavior is explicitly enabled, when a proxy server is part of a net-
work firewall it should not forward the names and ports of hosts behind the firewall,
because knowledge of network architecture behind a firewall might be of use to a
malicious party.*
Figure 6-22. HTTP/FTP gateway generates Via headers, logging the received protocol (FTP)
* Malicious people can use the names of computers and version numbers to learn about the network architec-
ture behind a security perimeter. This information might be helpful in security attacks. In addition, the
names of computers might be clues to private projects within an organization.
HTTP request message sent to proxy
GET ftp://http-guide.com/pub/welcome.txt HTTP/1.0
Client http-guide.com
FTP server
HTTP/1.0 200 OK
Date: Sun, 11 Nov 2001 21:01:59 GMT
Via: FTP/1.0 proxy.irenes-isp.net (Traffic-Server/5.0.1-17882 [cMsf])
Last-modified: Sun, 11 Nov 2001 21:05:24 GMT
Content-type: text/plain
Hi there. This is an FTP server.
HTTP response message
proxy.irenes-isp.net
(HTTP/1.0)
FTP request
FTP response
Tracing Messages |155
If Via node-name forwarding is not enabled, proxies that are part of a security perim-
eter should replace the hostname with an appropriate pseudonym for that host. Gen-
erally, though, proxies should try to retain a Via waypoint entry for each proxy
server, even if the real name is obscured.
For organizations that have very strong privacy requirements for obscuring the
design and topology of internal network architectures, a proxy may combine an
ordered sequence of Via waypoint entries (with identical received-protocol values)
into a single, joined entry. For example:
Via: 1.0 foo, 1.1 devirus.company.com, 1.1 access-logger.company.com
could be collapsed to:
Via: 1.0 foo, 1.1 concealed-stuff
Don’t combine multiple entries unless they all are under the same organizational
control and the hosts already have been replaced by pseudonyms. Also, don’t com-
bine entries that have different received-protocol values.
The TRACE Method
Proxy servers can change messages as the messages are forwarded. Headers are
added, modified, and removed, and bodies can be converted to different formats. As
proxies become more sophisticated, and more vendors deploy proxy products,
interoperability problems increase. To easily diagnose proxy networks, we need a
way to conveniently watch how messages change as they are forwarded, hop by hop,
through the HTTP proxy network.
HTTP/1.1’s TRACE method lets you trace a request message through a chain of
proxies, observing what proxies the message passes through and how each proxy
modifies the request message. TRACE is very useful for debugging proxy flows.*
When the TRACE request reaches the destination server,†the entire request mes-
sage is reflected back to the sender, bundled up in the body of an HTTP response
(see Figure 6-23). When the TRACE response arrives, the client can examine the
exact message the server received and the list of proxies through which it passed (in
the Via header). The TRACE response has Content-Type message/http and a 200
OK status.
Max-Forwards
Normally, TRACE messages travel all the way to the destination server, regardless of
the number of intervening proxies. You can use the Max-Forwards header to limit
* Unfortunately, it isn’t widely implemented yet.
† The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of
zero (0) in the request.
156 |Chapter 6: Proxies
the number of proxy hops for TRACE and OPTIONS requests, which is useful for
testing a chain of proxies forwarding messages in an infinite loop or for checking the
effects of particular proxy servers in the middle of a chain. Max-Forwards also limits
the forwarding of OPTIONS messages (see “Proxy Interoperation”).
The Max-Forwards request header field contains a single integer indicating the
remaining number of times this request message may be forwarded (Figure 6-24). If
the Max-Forwards value is zero (Max-Forwards: 0), the receiver must reflect the
TRACE message back toward the client without forwarding it further, even if the
receiver is not the origin server.
If the received Max-Forwards value is greater than zero, the forwarded message must
contain an updated Max-Forwards field with a value decremented by one. All prox-
ies and gateways should support Max-Forwards. You can use Max-Forwards to view
the request at any hop in a proxy chain.
Proxy Authentication
Proxies can serve as access-control devices. HTTP defines a mechanism called proxy
authentication that blocks requests for content until the user provides valid access-
permission credentials to the proxy:
• When a request for restricted content arrives at a proxy server, the proxy server
can return a 407 Proxy Authorization Required status code demanding access
Figure 6-23. TRACE response reflects back the received request message
Proxy 1
(proxy.irenes-isp net)
Client Server
(www.joes-hardware.com)
Proxy 2
(p1127.att net)
Proxy 3
(cache.joes-hardware.com)
TRACE /index.html HTTP/1.1
Host: www.joes-hardware.com
Accept: text/html
TRACE request
HTTP/1.1 200 OK
Content-Type: message/http
Content-Length: 269
Via: 1.1 cache.joes-hardware.com, 1.1 p1127.att.net, 1.1 proxy.irenes-isp.net
TRACE /index.html HTTP/1.1
Host: www.joes-hardware.com
Accept: text/html
Via: 1.1 proxy.irenes-isp.net, 1.1 p1127.att.net, 1.1 cache.joes-hardware.com
X-Magic-CDN-Thingy: 134-AF-0003
Cookie: access-isp="Irene’s ISP, California"
Client-ip: 209.134.49.32
TRACE response
Received request
Proxy Interoperation |157
credentials, accompanied by a Proxy-Authenticate header field that describes
how to provide those credentials (Figure 6-25b).
• When the client receives the 407 response, it attempts to gather the required cre-
dentials, either from a local database or by prompting the user.
• Once the credentials are obtained, the client resends the request, providing the
required credentials in a Proxy-Authorization header field.
• If the credentials are valid, the proxy passes the original request along the chain
(Figure 6-25c); otherwise, another 407 reply is sent.
Proxy authentication generally does not work well when there are multiple proxies in
a chain, each participating in authentication. People have proposed enhancements to
HTTP to associate authentication credentials with particular waypoints in a proxy
chain, but those enhancements have not been widely implemented.
Be sure to read Chapter 12 for a detailed explanation of HTTP’s authentication
mechanisms.
Proxy Interoperation
Clients, servers, and proxies are built by multiple vendors, to different versions of the
HTTP specification. They support various features and have different bugs. Proxy
servers need to intermediate between client-side and server-side devices, which may
implement different protocols and have troublesome quirks.
Figure 6-24. You can limit the forwarding hop count with the Max-Forwards header field
Proxy 1
(proxy.irenes-isp.net)
Client Server
(www.joes-hardware.com)
Proxy 2
(p1127.att.net)
Proxy 3
(cache.joes-hardware.com)
TRACE /index.html HTTP/1.1
Host: www.joes-hardware.com
Max-Forwards: 2
Accept: text/html
TRACE request
HTTP/1.1 200 OK
Content-Type: message/http
Content-Length: 269
Via: 1.1 p1127.att.net, 1.1 proxy.irenes-isp.net
TRACE /index.html HTTP/1.1
Host: www.joes-hardware.com
Accept: text/html
Via: 1.1 proxy.irenes-isp.net, 1.1 p1127.att.net
X-Magic-CDN-Thingy: 134-AF-0003
Cookie: access-isp="Irene’s ISP, California"
Client-ip: 209.134.49.32
TRACE response
Received request
Max-Forwards= 1 Max-Forwards= 0
158 |Chapter 6: Proxies
Handling Unsupported Headers and Methods
The proxy server may not understand all the header fields that pass through it. Some
headers may be newer than the proxy itself; others may be customized header fields
unique to a particular application. Proxies must forward unrecognized header fields
and must maintain the relative order of header fields with the same name.*Similarly,
if a proxy is unfamiliar with a method, it should try to forward the message to the
next hop, if possible.
Proxies that cannot tunnel unsupported methods may not be viable in most net-
works today, because Hotmail access through Microsoft Outlook makes extensive
use of HTTP extension methods.
Figure 6-25. Proxies can implement authentication to control access to content
* Multiple message header fields with the same field name may be present in a message, but if they are, they
must be able to be equivalently combined into a comma-separated list. The order in which header fields with
the same field name are received is therefore significant to the interpretation of the combined field value, so
a proxy can’t change the relative order of these same-named field values when it forwards a message.
Client Server
(a) GET http://server.com/secret.jpg HTTP/1.0
Client Server
(b) HTTP/1.o 407 Proxy Authorization Required
Proxy-Authenticate: Basic realm="Secure Stuff"
Client
(c) GET http://server.com/secret.jpg HTTP/1.0
Proxy-Authorization: Basic YnJpOmZvbw==
Client Server
(d) HTTP/1.0 200 OK
Content-type: image/jpeg
...<image data included>...
Access control
proxy
Server
Super secret
image
Access control
proxy
Access control
proxy
Access control
proxy
Proxy Interoperation |159
OPTIONS: Discovering Optional Feature Support
The HTTP OPTIONS method lets a client (or proxy) discover the supported func-
tionality (for example, supported methods) of a web server or of a particular resource
on a web server (Figure 6-26). Clients can use OPTIONS to determine a server’s
capabilities before interacting with the server, making it easier to interoperate with
proxies and servers of different feature levels.
If the URI of the OPTIONS request is an asterisk (*), the request pertains to the
entire server’s supported functionality. For example:
OPTIONS * HTTP/1.1
If the URI is a real resource, the OPTIONS request inquires about the features avail-
able to that particular resource:
OPTIONS http://www.joes-hardware.com/index.html HTTP/1.1
On success, the OPTIONS method returns a 200 OK response that includes various
header fields that describe optional features that are supported on the server or avail-
able to the resource. The only header field that HTTP/1.1 specifies in the response is
the Allow header, which describes what methods are supported by the server (or
particular resource on the server).*OPTIONS allows an optional response body with
more information, but this is undefined.
The Allow Header
The Allow entity header field lists the set of methods supported by the resource iden-
tified by the request URI, or the entire server if the request URI is *. For example:
Allow: GET, HEAD, PUT
The Allow header can be used as a request header to recommend the methods to be
supported by the new resource. The server is not required to support these methods
Figure 6-26. Using OPTIONS to find a server’s supported methods
* Not all resources support every method. For example, a CGI script query may not support a file PUT, and a
static HTML file wouldn’t accept a POST method.
Client Proxy Server
OPTIONS * HTTP/1.1
HTTP/1.1 200 OK
Allow: GET,PUT,POST,HEAD,TRACE,OPTIONS
160 |Chapter 6: Proxies
and should include an Allow header in the matching response, listing the actual sup-
ported methods.
A proxy can’t modify the Allow header field even if it does not understand all the
methods specified, because the client might have other paths to talk to the origin
server.
For More Information
For more information, refer to:
http://www.w3.org/Protocols/rfc2616/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol,” by R. Fielding, J. Gettys, J. Mogul, H.
Frystyk, L. Mastinter, P. Leach, and T. Berners-Lee.
http://search.ietf.org/rfc/rfc3040.txt
RFC 3040, “Internet Web Replication and Caching Taxonomy.”
Web Proxy Servers
Ari Luotonen, Prentice Hall Computer Books.
http://search.ietf.org/rfc/rfc3143.txt
RFC 3143, “Known HTTP Proxy/Caching Problems.”
Web Caching
Duane Wessels, O’Reilly & Associates, Inc.
161
CHAPTER 7
Caching
Web caches are HTTP devices that automatically keep copies of popular docu-
ments. When a web request arrives at a cache, if a local “cached” copy is available,
the document is served from the local storage instead of from the origin server.
Caches have the following benefits:
• Caches reduce redundant data transfers, saving you money in network charges.
• Caches reduce network bottlenecks. Pages load faster without more bandwidth.
• Caches reduce demand on origin servers. Servers reply faster and avoid overload.
• Caches reduce distance delays, because pages load slower from farther away.
In this chapter, we explain how caches improve performance and reduce cost, how
to measure their effectiveness, and where to place caches to maximize impact. We
also explain how HTTP keeps cached copies fresh and how caches interact with
other caches and servers.
Redundant Data Transfers
When multiple clients access a popular origin server page, the server transmits the
same document multiple times, once to each client. The same bytes travel across the
network over and over again. These redundant data transfers eat up expensive net-
work bandwidth, slow down transfers, and overload web servers. With caches, the
cache keeps a copy of the first server response. Subsequent requests can be fulfilled
from the cached copy, reducing wasteful, duplicate traffic to and from origin servers.
Bandwidth Bottlenecks
Caches also can reduce network bottlenecks. Many networks provide more band-
width to local network clients than to remote servers (Figure 7-1). Clients access serv-
ers at the speed of the slowest network on the way. If a client gets a copy from a cache
on a fast LAN, caching can boost performance—especially for larger documents.
162 |Chapter 7: Caching
In Figure 7-1, it might take 30 seconds for a user in the San Francisco branch of Joe’s
Hardware, Inc. to download a 5-MB inventory file from the Atlanta headquarters,
across the 1.4-Mbps T1 Internet connection. If the document was cached in the San
Francisco office, a local user might be able to get the same document in less than a
second across the Ethernet connection.
Table 7-1 shows how bandwidth affects transfer time for a few different network
speeds and a few different sizes of documents. Bandwidth causes noticeable delays
for larger documents, and the speed difference between different network types is
dramatic.*A 56-Kbps modem would take 749 seconds (over 12 minutes) to transfer a
5-MB file that could be transported in under a second across a fast Ethernet LAN.
Figure 7-1. Limited wide area bandwidth creates a bottleneck that caches can improve
* This table shows just the effect of network bandwidth on transfer time. It assumes 100% network efficiency
and no network or application processing latencies. In this way, the delay is a lower bound. Real delays will
be larger, and the delays for small objects will be dominated by non-bandwidth overheads.
Table 7-1. Bandwidth-imposed transfer time delays, idealized (time in seconds)
Large HTML (15 KB) JPEG (40 KB) Large JPEG (150 KB) Large file (5 MB)
Dialup modem (56 Kbit/sec) 2.19 5.85 21.94 748.98
DSL (256 Kbit/sec) .48 1.28 4.80 163.84
T1 (1.4 Mbit/sec) .09 .23 .85 29.13
Slow Ethernet (10 Mbit/sec) .01 .03 .12 4.19
DS3 (45 Mbit/sec) .00 .01 .03 .93
Fast Ethernet (100 Mbit/sec) .00 .00 .01 .42
Atlanta corporate headquarters
San Francisco branch office
Client
Server
Cache
Fast connection to cache
(100 Mbit/sec ethernet)
Slow WAN connection to server
(1.4 Mbit/sec T1)
Distance Delays |163
Flash Crowds
Caching is especially important to break up flash crowds. Flash crowds occur when a
sudden event (such as breaking news, a bulk email announcement, or a celebrity
event) causes many people to access a web document at nearly the same time
(Figure 7-2). The resulting redundant traffic spike can cause a catastrophic collapse
of networks and web servers.
When the “Starr Report” detailing Kenneth Starr’s investigation of U.S. President
Clinton was released to the Internet on September 11, 1998, the U.S. House of Rep-
resentatives web servers received over 3 million requests per hour, 50 times the aver-
age server load. One news web site, CNN.com, reported an average of over 50,000
requests every second to its servers.
Distance Delays
Even if bandwidth isn’t a problem, distance might be. Every network router adds
delays to Internet traffic. And even if there are not many routers between client and
server, the speed of light alone can cause a significant delay.
The direct distance from Boston to San Francisco is about 2,700 miles. In the very best
case, at the speed of light (186,000 miles/sec), a signal could travel from Boston to San
Francisco in about 15 milliseconds and complete a round trip in 30 milliseconds.*
Figure 7-2. Flash crowds can overload web servers
* In reality, signals travel at somewhat less than the speed of light, so distance delays are even worse.
Atlanta
San
Francisco
Los Angeles
Boston
Chicago
Flash crowd
164 |Chapter 7: Caching
Say a web page contains 20 small images, all located on a server in San Francisco. If a
client in Boston opens four parallel connections to the server, and keeps the connec-
tions alive, the speed of light alone contributes almost 1/4 second (240 msec) to the
download time (Figure 7-3). If the server is in Tokyo (6,700 miles from Boston), the
delay grows to 600 msec. Moderately complicated web pages can incur several sec-
onds of speed-of-light delays.
Placing caches in nearby machine rooms can shrink document travel distance from
thousands of miles to tens of yards.
Hits and Misses
So caches can help. But a cache doesn’t store a copy of every document in the
world.*
Figure 7-3. Speed of light can cause significant delays, even with parallel, keep-alive connections
* Few folks can afford to buy a cache big enough to hold all the Web’s documents. And even if you could afford
gigantic “whole-Web caches,” some documents change so frequently that they won’t be fresh in many caches.
Connection 1 Connection 2 Connection 3 Connection 4
30 msec
Connect request
OK
30 msec
GET web page
Web page
30 msec
GET image 1
image 1
30 msec
GET image 2
image 2
30 msec
GET image 6
image 6
30 msec
GET image 10
image 10
30 msec
GET image 14
image 14
30 msec
GET image 18
image 18
30 msec
Connect request
OK
30 msec
GET image 3
image 3
30 msec
GET image 7
image 7
30 msec
GET image 11
image 11
30 msec
GET image 15
image 15
30 msec
GET image 19
image 19
30 msec
Connect request
OK
30 msec
GET image 4
image 4
30 msec
GET image 8
image 8
30 msec
GET image 12
image 12
30 msec
GET image 16
image 16
30 msec
GET image 20
image 20
30 msec
Connect request
OK
30 msec
GET image 5
image 5
30 msec
GET image 9
image 9
30 msec
GET image 13
image 13
30 msec
GET image 17
image 17
30 msec
GET image 21
image 21
240 msec
Speed of light delay
Client in Boston
Server in San Francisco
speed of light 30 msec round trip
Hits and Misses |165
Some requests that arrive at a cache can be served from an available copy. This is
called a cache hit (Figure 7-4a). Other requests arrive at a cache only to be forwarded
to the origin server, because no copy is available. This is called a cache miss
(Figure 7-4b).
Revalidations
Because the origin server content can change, caches have to check every now and
then that their copies are still up-to-date with the server. These “freshness checks”
are called HTTP revalidations (Figure 7-4c). To make revalidations efficient, HTTP
defines special requests that can quickly check if content is still fresh, without fetch-
ing the entire object from the server.
A cache can revalidate a copy any time it wants, and as often as it wants. But because
caches often contain millions of documents, and because network bandwidth is
scarce, most caches revalidate a copy only when it is requested by a client and when
the copy is old enough to warrant a check. We’ll explain the HTTP rules for fresh-
ness checking later in the chapter.
When a cache needs to revalidate a cached copy, it sends a small revalidation request
to the origin server. If the content hasn’t changed, the server responds with a tiny
304 Not Modified response. As soon as the cache learns the copy is still valid, it
marks the copy temporarily fresh again and serves the copy to the client
(Figure 7-5a). This is called a revalidate hit oraslow hit. It’s slower than a pure cache
hit, because it does need to check with the origin server, but it’s faster than a cache
miss, because no object data is retrieved from the server.
Figure 7-4. Cache hits, misses, and revalidations
(a ) Cache hit
Client ServerCache
Cache
object
(b ) Cache miss
Client ServerCache
(c ) Cache revalidate hit
Client ServerCache
Cache
object
Server
object
Freshness check
“Still fresh”
Server
object
166 |Chapter 7: Caching
HTTP gives us a few tools to revalidate cached objects, but the most popular is the
If-Modified-Since header. When added to a GET request, this header tells the server
to send the object only if it has been modified since the time the copy was cached.
Here is what happens when a GET If-Modified-Since request arrives at the server in
three circumstances—when the server content is not modified, when the server con-
tent has been changed, and when the object on the server is deleted:
Revalidate hit
If the server object isn’t modified, the server sends the client a small HTTP 304
Not Modified response. This is depicted in Figure 7-6.
Revalidate miss
If the server object is different from the cached copy, the server sends the client a
normal HTTP 200 OK response, with the full content.
Figure 7-5. Successful revalidations are faster than cache misses; failed revalidations are nearly
identical to misses
Figure 7-6. HTTP uses If-Modified-Since header for revalidation
Server
object
(a) Revalidate hit (slow hit)
Client ServerCache
Freshness check
“Still fresh”
(b) Revalidate miss
Client ServerCache
Freshness check
Server
object
Server object same as cached copy
Cached copy is out of date
Cache
object
Server
GET /announce.html HTTP/1.0
If-Modified-Since: Sat, 29 Jun 2002, 14:30:00 GMT
HTTP/1.0 304 Not Modified
Date: Wed, 03 Jul 2002, 19:18:23 GMT
Content-type: text/plain
Content-length: 67
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
Cache
(browser cache or
proxy cache)
Revalidate request with If-Modified-Since
“Still fresh” response
Hits and Misses |167
Object deleted
If the server object has been deleted, the server sends back a 404 Not Found
response, and the cache deletes its copy.
Hit Rate
The fraction of requests that are served from cache is called the cache hit rate (or
cache hit ratio),*or sometimes the document hit rate (or document hit ratio). The hit
rate ranges from 0 to 1 but is often described as a percentage, where 0% means that
every request was a miss (had to get the document across the network), and 100%
means every request was a hit (had a copy in the cache).†
Cache administrators would like the cache hit rate to approach 100%. The actual hit
rate you get depends on how big your cache is, how similar the interests of the cache
users are, how frequently the cached data is changing or personalized, and how the
caches are configured. Hit rate is notoriously difficult to predict, but a hit rate of
40% is decent for a modest web cache today. The nice thing about caches is that
even a modest-sized cache may contain enough popular documents to significantly
improve performance and reduce traffic. Caches work hard to ensure that useful con-
tent stays in the cache.
Byte Hit Rate
Document hit rate doesn’t tell the whole story, though, because documents are not
all the same size. Some large objects might be accessed less often but contribute
more to overall data traffic, because of their size. For this reason, some people pre-
fer the byte hit rate metric (especially those folks who are billed for each byte of
traffic!).
The byte hit rate represents the fraction of all bytes transferred that were served from
cache. This metric captures the degree of traffic savings. A byte hit rate of 100%
means every byte came from the cache, and no traffic went out across the Internet.
Document hit rate and byte hit rate are both useful gauges of cache performance.
Document hit rate describes how many web transactions are kept off the outgoing
network. Because transactions have a fixed time component that can often be large
(setting up a TCP connection to a server, for example), improving the document hit
rate will optimize for overall latency (delay) reduction. Byte hit rate describes how
many bytes are kept off the Internet. Improving the byte hit rate will optimize for
bandwidth savings.
* The term “hit ratio” probably is better than “hit rate,” because “hit rate” mistakenly suggests a time factor.
However, “hit rate” is in common use, so we use it here.
† Sometimes people include revalidate hits in the hit rate, but other times hit rate and revalidate hit rate are
measured separately. When you are examining hit rates, be sure you know what counts as a “hit.”
168 |Chapter 7: Caching
Distinguishing Hits and Misses
Unfortunately, HTTP provides no way for a client to tell if a response was a cache hit
or an origin server access. In both cases, the response code will be 200 OK, indicat-
ing that the response has a body. Some commercial proxy caches attach additional
information to Via headers to describe what happened in the cache.
One way that a client can usually detect if the response came from a cache is to use
the Date header. By comparing the value of the Date header in the response to the
current time, a client can often detect a cached response by its older date value.
Another way a client can detect a cached response is the Age header, which tells how
old the response is (see “Age” in Appendix C).
Cache Topologies
Caches can be dedicated to a single user or shared between thousands of users. Dedi-
cated caches are called private caches. Private caches are personal caches, containing
popular pages for a single user (Figure 7-7a). Shared caches are called public caches.
Public caches contain the pages popular in the user community (Figure 7-7b).
Private Caches
Private caches don’t need much horsepower or storage space, so they can be made
small and cheap. Web browsers have private caches built right in—most browsers
cache popular documents in the disk and memory of your personal computer and
allow you to configure the cache size and settings. You also can peek inside the
browser caches to see what they contain. For example, with Microsoft Internet
Figure 7-7. Public and private caches
Private cache
Client
Internet
Web server
Public cache
Client Internet
Web server
(a ) Accessing private cache
Client
(b ) Accessing shared public cache
Cache Topologies |169
Explorer, you can get the cache contents from the Tools ➝Internet Options... dia-
log box. MSIE calls the cached documents “Temporary Files” and lists them in a file
display, along with the associated URLs and document expiration times. You can
view Netscape Navigator’s cache contents through the special URL about:cache,
which gives you a “Disk Cache statistics” page showing the cache contents.
Public Proxy Caches
Public caches are special, shared proxy servers called caching proxy servers or, more
commonly, proxy caches (proxies were discussed in Chapter 6). Proxy caches serve
documents from the local cache or contact the server on the user’s behalf. Because a
public cache receives accesses from multiple users, it has more opportunity to elimi-
nate redundant traffic.*
In Figure 7-8a, each client redundantly accesses a new, “hot” document (not yet in
the private cache). Each private cache fetches the same document, crossing the net-
work multiple times. With a shared, public cache, as in Figure 7-8b, the cache needs
to fetch the popular object only once, and it uses the shared copy to service all
requests, reducing network traffic.
Proxy caches follow the rules for proxies described in Chapter 6. You can configure
your browser to use a proxy cache by specifying a manual proxy or by configuring a
proxy auto-configuration file (see “Client Proxy Configuration: Manual” in Chapter 6).
You also can force HTTP requests through caches without configuring your browser
by using intercepting proxies (see Chapter 20).
Proxy Cache Hierarchies
In practice, it often makes sense to deploy hierarchies of caches, where cache misses in
smaller caches are funneled to larger parent caches that service the leftover “distilled”
traffic. Figure 7-9 shows a two-level cache hierarchy.†The idea is to use small, inex-
pensive caches near the clients and progressively larger, more powerful caches up the
hierarchy to hold documents shared by many users.‡
Hopefully, most users will get cache hits on the nearby, level-1 caches (as shown in
Figure 7-9a). If not, larger parent caches may be able to handle their requests
(Figure 7-9b). For deep cache hierarchies it’s possible to go through long chains of
* Because a public cache caches the diverse interests of the user community, it needs to be large enough to hold
a set of popular documents, without being swept clean by individual user interests.
† If the clients are browsers with browser caches, Figure 7-9 technically shows a three-level cache hierarchy.
‡ Parent caches may need to be larger, to hold the documents popular across more users, and higher-
performance, because they receive the aggregate traffic of many children, whose interests may be diverse.
170 |Chapter 7: Caching
caches, but each intervening proxy does impose some performance penalty that can
become noticeable as the proxy chain becomes long.*
Cache Meshes, Content Routing, and Peering
Some network architects build complex cache meshes instead of simple cache hierar-
chies. Proxy caches in cache meshes talk to each other in more sophisticated ways,
and make dynamic cache communication decisions, deciding which parent caches to
talk to, or deciding to bypass caches entirely and direct themselves to the origin
server. Such proxy caches can be described as content routers, because they make
routing decisions about how to access, manage, and deliver content.
Caches designed for content routing within cache meshes may do all of the follow-
ing (among other things):
• Select between a parent cache or origin server dynamically, based on the URL.
• Select a particular parent cache dynamically, based on the URL.
Figure 7-8. Shared, public caches can decrease network traffic
* In practice, network architects try to limit themselves to two or three proxies in a row. However, a new gen-
eration of high-performance proxy servers may make proxy-chain length less of an issue.
Client
(a) Redundant accesses from private caches
Client
Client
Internet
Server
Client
(b) Shared caches can reduce traffic
Client
Client
Internet
Server
Cache
Cache Processing Steps |171
• Search caches in the local area for a cached copy before going to a parent cache.
• Allow other caches to access portions of their cached content, but do not permit
Internet transit through their cache.
These more complex relationships between caches allow different organizations to
peer with each other, connecting their caches for mutual benefit. Caches that pro-
vide selective peering support are called sibling caches (Figure 7-10). Because HTTP
doesn’t provide sibling cache support, people have extended HTTP with protocols,
such as the Internet Cache Protocol (ICP) and the HyperText Caching Protocol
(HTCP). We’ll talk about these protocols in Chapter 20.
Cache Processing Steps
Modern commercial proxy caches are quite complicated. They are built to be very
high-performance and to support advanced features of HTTP and other technologies.
But, despite some subtle details, the basic workings of a web cache are mostly simple.
A basic cache-processing sequence for an HTTP GET message consists of seven steps
(illustrated in Figure 7-11):
1. Receiving—Cache reads the arriving request message from the network.
2. Parsing—Cache parses the message, extracting the URL and headers.
Figure 7-9. Accessing documents in a two-level cache hierarchy
X X
X
X
X
X
Origin server
Level-2 cache
Wide area
network
Regional network
Level-1
cache
(a) Level-1 cache hit
Origin server
Level-2 cache
Wide area
network
Regional network
Level-1
cache
(b) Level-2 cache hit
Origin server
Level-2 cache
Wide area
network
Regional network
Level-1
cache
(c) Level-2 cache miss
172 |Chapter 7: Caching
3. Lookup—Cache checks if a local copy is available and, if not, fetches a copy
(and stores it locally).
4. Freshness check—Cache checks if cached copy is fresh enough and, if not, asks
server for any updates.
5. Response creation—Cache makes a response message with the new headers and
cached body.
6. Sending—Cache sends the response back to the client over the network.
7. Logging—Optionally, cache creates a log file entry describing the transaction.
Step 1: Receiving
In Step 1, the cache detects activity on a network connection and reads the incoming
data. High-performance caches read data simultaneously from multiple incoming con-
nections and begin processing the transaction before the entire message has arrived.
Step 2: Parsing
Next, the cache parses the request message into pieces and places the header parts in
easy-to-manipulate data structures. This makes it easier for the caching software to
process the header fields and fiddle with them.*
Figure 7-10. Sibling caches
* The parser also is responsible for normalizing the parts of the header so that unimportant differences, like
capitalization or alternate date formats, all are viewed equivalently. Also, because some request messages
contain a full absolute URL and other request messages contain a relative URL and Host header, the parser
typically hides these details (see “Relative URLs” in Chapter 2).
X
Origin server
Wide area
network
Organization A
B’s access point
Organization B
A’s access point
Sibling
Y
X
Cache Processing Steps |173
Step 3: Lookup
In Step 3, the cache takes the URL and checks for a local copy. The local copy
might be stored in memory, on a local disk, or even in another nearby computer.
Professional-grade caches use fast algorithms to determine whether an object is
available in the local cache. If the document is not available locally, it can be fetched
from the origin server or a parent proxy, or return a failure, based on the situation
and configuration.
The cached object contains the server response body and the original server response
headers, so the correct server headers can be returned during a cache hit. The cached
object also includes some metadata, used for bookkeeping how long the object has
been sitting in the cache, how many times it was used, etc.*
Step 4: Freshness Check
HTTP lets caches keep copies of server documents for a period of time. During this
time, the document is considered “fresh” and the cache can serve the document with-
out contacting the server. But once the cached copy has sat around for too long, past
the document’s freshness limit, the object is considered “stale,” and the cache needs to
Figure 7-11. Processing a fresh cache hit
* Sophisticated caches also keep a copy of the original client response headers that yielded the server response,
for use in HTTP/1.1 content negotiation (see Chapter 17).
Client Server
GET /www.joes-hardware.com/index.html HTTP/1.1
User-agent: Superbrowser 2.0
Host: www.joes-hardware.com
Accept: *.*
(1) Receive HTTP request message
(2) Parse message (3) In cache?
Cache
Server
headers
Body
YES
Server
headers
Body
NEW
headers
Body
(4) Is fresh?
(5) Create response headers
HTTP/1.1 200 OK
Content-length: 2140
Content-type: text/html
Cache-control: max-age=86400
Age: 21562
Via: ...
<HEAD><TITLE>Joe’s Hardware Home Page</TITLE></HEAD>
<BODY><H1>Welcome to Joe’s Hardware</H1>...
YES
(6) Send response
174 |Chapter 7: Caching
revalidate with the server to check for any document changes before serving it. Com-
plicating things further are any request headers that a client sends to a cache, which
themselves can force the cache to either revalidate or avoid validation altogether.
HTTP has a set of very complicated rules for freshness checking, made worse by the
large number of configuration options cache products support and by the need to
interoperate with non-HTTP freshness standards. We’ll devote most of the rest of
this chapter to explaining freshness calculations.
Step 5: Response Creation
Because we want the cached response to look like it came from the origin server, the
cache uses the cached server response headers as the starting point for the response
headers. These base headers are then modified and augmented by the cache.
The cache is responsible for adapting the headers to match the client. For example,
the server may return an HTTP/1.0 response (or even an HTTP/0.9 response), while
the client expects an HTTP/1.1 response, in which case the cache must translate the
headers accordingly. Caches also insert cache freshness information (Cache-Control,
Age, and Expires headers) and often include a Via header to note that a proxy cache
served the request.
Note that the cache should not adjust the Date header. The Date header represents
the date of the object when it was originally generated at the origin server.
Step 6: Sending
Once the response headers are ready, the cache sends the response back to the cli-
ent. Like all proxy servers, a proxy cache needs to manage the connection with the
client. High-performance caches work hard to send the data efficiently, often avoid-
ing copying the document content between the local storage and the network I/O
buffers.
Step 7: Logging
Most caches keep log files and statistics about cache usage. After each cache transac-
tion is complete, the cache updates statistics counting the number of cache hits and
misses (and other relevant metrics) and inserts an entry into a log file showing the
request type, URL, and what happened.
The most popular cache log formats are the Squid log format and the Netscape
extended common log format, but many cache products allow you to create custom
log files. We discuss log file formats in detail in Chapter 21.
Keeping Copies Fresh |175
Cache Processing Flowchart
Figure 7-12 shows, in simplified form, how a cache processes a request to GET a
URL.*
Keeping Copies Fresh
Cached copies might not all be consistent with the documents on the server. After
all, documents do change over time. Reports might change monthly. Online newspa-
pers change daily. Financial data may change every few seconds. Caches would be
useless if they always served old data. Cached data needs to maintain some consis-
tency with the server data.
HTTP includes simple mechanisms to keep cached data sufficiently consistent with
servers, without requiring servers to remember which caches have copies of their
documents. HTTP calls these simple mechanisms document expiration and server
revalidation.
Document Expiration
HTTP lets an origin server attach an “expiration date” to each document, using spe-
cial HTTP Cache-Control and Expires headers (Figure 7-13). Like an expiration date
on a quart of milk, these headers dictate how long content should be viewed as fresh.
Figure 7-12. Cache GET request flowchart
* The revalidation and fetching of a resource as outlined in Figure 7-12 can be done in one step with a condi-
tional request (see “Revalidation with Conditional Methods”).
Revalidate with server Revalidated? Fetch from server
Store into cache
Serve to client
Update freshness
of cached document
Fresh enough?
Cached?
Request arrives
no
yes
no
yes yes
no
176 |Chapter 7: Caching
Until a cache document expires, the cache can serve the copy as often as it wants,
without ever contacting the server—unless, of course, a client request includes
headers that prevent serving a cached or unvalidated resource. But, once the cached
document expires, the cache must check with the server to ask if the document has
changed and, if so, get a fresh copy (with a new expiration date).
Expiration Dates and Ages
Servers specify expiration dates using the HTTP/1.0+ Expires or the HTTP/1.1
Cache-Control: max-age response headers, which accompany a response body. The
Expires and Cache-Control: max-age headers do basically the same thing, but the
newer Cache-Control header is preferred, because it uses a relative time instead of an
absolute date. Absolute dates depend on computer clocks being set correctly.
Table 7-2 lists the expiration response headers.
Let’s say today is June 29, 2002 at 9:30 am Eastern Standard Time (EST), and Joe’s
Hardware store is getting ready for a Fourth of July sale (only five days away). Joe
wants to put a special web page on his web server and set it to expire at midnight
EST on the night of July 5, 2002. If Joe’s server uses the older-style Expires headers,
the server response message (Figure 7-13a) might include this header:*
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
Figure 7-13. Expires and Cache Control headers
Table 7-2. Expiration response headers
Header Description
Cache-Control: max-age The max-age value defines the maximum age of the document—the maximum legal elapsed
time (in seconds) from when a document is first generated to when it can no longer be considered
fresh enough to serve.
Cache-Control: max-age=484200
Expires Specifies an absolute expiration date. If the expiration date is in the past, the document is no
longer fresh.
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
* Note that all HTTP dates and times are expressed in Greenwich Mean Time (GMT). GMT is the time at the
prime meridian (0˚ longitude) that passes through Greenwich, UK. GMT is five hours ahead of U.S. Eastern
Standard Time, so midnight EST is 05:00 GMT.
HTTP/1.0 200 OK
Date: Sat, 29 Jun 2002, 14:30:00 GMT
Content-type: text/plain
Content-length: 67
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
Independence Day sale at Joe's Hardware
Come shop with us today!
(a) Expires header
HTTP/1.0 200 OK
Date: Sat, 29 Jun 2002, 14:30:00 GMT
Content-type: text/plain
Content-length: 67
Cache-Control: max-age=484200
Independence Day sale at Joe's Hardware
Come shop with us today!
(b) Cache-Control: max-age header
Keeping Copies Fresh |177
If Joe’s server uses the newer Cache-Control: max-age headers, the server response
message (Figure 7-13b) might contain this header:
Cache-Control: max-age=484200
In case that wasn’t immediately obvious, 484,200 is the number of seconds between
the current date, June 29, 2002 at 9:30 am EST, and the sale end date, July 5, 2002 at
midnight. There are 134.5 hours (about 5 days) until the sale ends. With 3,600 sec-
onds in each hour, that leaves 484,200 seconds until the sale ends.
Server Revalidation
Just because a cached document has expired doesn’t mean it is actually different
from what’s living on the origin server; it just means that it’s time to check. This is
called “server revalidation,” meaning the cache needs to ask the origin server
whether the document has changed:
• If revalidation shows the content has changed, the cache gets a new copy of the
document, stores it in place of the old data, and sends the document to the client.
• If revalidation shows the content has not changed, the cache only gets new head-
ers, including a new expiration date, and updates the headers in the cache.
This is a nice system. The cache doesn’t have to verify a document’s freshness for
every request—it has to revalidate with the server only once the document has
expired. This saves server traffic and provides better user response time, without
serving stale content.
The HTTP protocol requires a correctly behaving cache to return one of the following:
• A cached copy that is “fresh enough”
• A cached copy that has been revalidated with the server to ensure it’s still fresh
• An error message, if the origin server to revalidate with is down*
• A cached copy, with an attached warning that it might be incorrect
Revalidation with Conditional Methods
HTTP’s conditional methods make revalidation efficient. HTTP allows a cache to
send a “conditional GET” to the origin server, asking the server to send back an
object body only if the document is different from the copy currently in the cache. In
this manner, the freshness check and the object fetch are combined into a single con-
ditional GET. Conditional GETs are initiated by adding special conditional headers to
GET request messages. The web server returns the object only if the condition is true.
* If the origin server is not accessible, but the cache needs to revalidate, the cache must return an error or a
warning describing the communication failure. Otherwise, pages from a removed server may live in network
caches for an arbitrary time into the future.
178 |Chapter 7: Caching
HTTP defines five conditional request headers. The two that are most useful for
cache revalidation are If-Modified-Since and If-None-Match.*All conditional head-
ers begin with the prefix “If-”. Table 7-3 lists the conditional response headers used
in cache revalidation.
If-Modified-Since: Date Revalidation
The most common cache revalidation header is If-Modified-Since. If-Modified-Since
revalidation requests often are called “IMS” requests. IMS requests instruct a server
to perform the request only if the resource has changed since a certain date:
• If the document was modified since the specified date, the If-Modified-Since
condition is true, and the GET succeeds normally. The new document is
returned to the cache, along with new headers containing, among other informa-
tion, a new expiration date.
• If the document was not modified since the specified date, the condition is false,
and a small 304 Not Modified response message is returned to the client, with-
out a document body, for efficiency.†Headers are returned in the response;
however, only the headers that need updating from the original need to be
returned. For example, the Content-Type header does not usually need to be
sent, since it usually has not changed. A new expiration date typically is sent.
The If-Modified-Since header works in conjunction with the Last-Modified server
response header. The origin server attaches the last modification date to served docu-
ments. When a cache wants to revalidate a cached document, it includes an If-Modi-
fied-Since header with the date the cached copy was last modified:
If-Modified-Since: <cached last-modified date>
* Other conditional headers include If-Unmodified-Since (useful for partial document transfers, when you
need to ensure the document is unchanged before you fetch another piece of it), If-Range (to support caching
of incomplete documents), and If-Match (useful for concurrency control when dealing with web servers).
Table 7-3. Two conditional headers used in cache revalidation
Header Description
If-Modified-Since:
<date>
Perform the requested method if the document has been modified since the specified date. This
is used in conjunction with the Last-Modified server response header, to fetch content only if
the content has been modified from the cached version.
If-None-Match: <tags> Instead of matching on last-modified date, the server may provide special tags (see “ETag” in
Appendix C) on the document that act like serial numbers. The If-None-Match header performs
the requested method if the cached tags differ from the tags in the server’s document.
† If an old server that doesn’t recognize the If-Modified-Since header gets the conditional request, it interprets
it as a normal GET. In this case, the system will still work, but it will be less efficient due to unnecessary
transmittal of unchanged document data.
Keeping Copies Fresh |179
If the content has changed in the meantime, the last modification date will be differ-
ent, and the origin server will send back the new document. Otherwise, the server
will note that the cache’s last-modified date matches the server document’s current
last-modified date, and it will return a 304 Not Modified response.
For example, as shown in Figure 7-14, if your cache revalidates Joe’s Hardware’s
Fourth of July sale announcement on July 3, you will receive back a Not Modified
response (Figure 7-14a). But if your cache revalidates the document after the sale
ends at midnight on July 5, the cache will receive a new document, because the
server content has changed (Figure 7-14b).
Note that some web servers don’t implement If-Modified-Since as a true date com-
parison. Instead, they do a string match between the IMS date and the last-modified
date. As such, the semantics behave as “if not last modified on this exact date”
instead of “if modified since this date.” This alternative semantic works fine for
Figure 7-14. If-Modified-Since revalidations return 304 if unchanged or 200 with new body if
changed
(a) If-Modified-Since successful revalidation
Client Server
GET /announce.html HTTP/1.0
If-Modified-Since: Sat, 29 Jun 2002, 14:30:00 GMT
Conditional request
HTTP/1.0 304 Not Modified
Date: Wed, 03 Jul 2002, 19:18:23 GMT
Expires: Fri, 05 Jul 2002, 14:30:00 GMT
Response
(b) If-Modified-Since failed revalidation
Client Server
GET /announce.html HTTP/1.0
If-Modified-Since: Sat, 29 Jun 2002, 14:30:00 GMT
Conditional request
HTTP/1.0 200 OK
Date: Fri, 05 Jul 2002, 17:54:40 GMT
Content-type: text/plain
Content-length: 124
Expires: Mon, 09 Sep 2002, 05:00:00 GMT
All exterior house paint on sale through
Labor Day. Just another reason for you
to shop this summer at Joe's Hardware!
Response
180 |Chapter 7: Caching
cache expiration, when you are using the last-modified date as a kind of serial num-
ber, but it prevents clients from using the If-Modified-Since header for true time-
based purposes.
If-None-Match: Entity Tag Revalidation
There are some situations when the last-modified date revalidation isn’t adequate:
• Some documents may be rewritten periodically (e.g., from a background pro-
cess) but actually often contain the same data. The modification dates will
change, even though the content hasn’t.
• Some documents may have changed, but only in ways that aren’t important
enough to warrant caches worldwide to reload the data (e.g., spelling or com-
ment changes).
• Some servers cannot accurately determine the last modification dates of their
pages.
• For servers that serve documents that change in sub-second intervals (e.g. real-
time monitors), the one-second granularity of modification dates might not be
adequate.
To get around these problems, HTTP allows you to compare document “version
identifiers” called entity tags (ETags). Entity tags are arbitrary labels (quoted strings)
attached to the document. They might contain a serial number or version name for
the document, or a checksum or other fingerprint of the document content.
When the publisher makes a document change, he can change the document’s entity
tag to represent this new version. Caches can then use the If-None-Match condi-
tional header to GET a new copy of the document if the entity tags have changed.
In Figure 7-15, the cache has a document with entity tag “v2.6”. It revalidates with
the origin server asking for a new object only if the tag “v2.6” no longer matches. In
Figure 7-15, the tag still matches, so a 304 Not Modified response is returned.
Figure 7-15. If-None-Match revalidates because entity tag still matches
Cache Server
GET /announce.html HTTP/1.0
If-None-Match: "v2.6"
Conditional request
HTTP/1.0 304 Not Modified
Date: Wed, 03 Jul 2002, 19:18:23 GMT
ETag: "v2.6"
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
Response
ETag: “v2.6”ETag: “v2.6”
Keeping Copies Fresh |181
If the entity tag on the server had changed (perhaps to “v3.0”), the server would
return the new content in a 200 OK response, along with the content and new ETag.
Several entity tags can be included in an If-None-Match header, to tell the server that
the cache already has copies of objects with those entity tags:
If-None-Match: "v2.6"
If-None-Match: "v2.4","v2.5","v2.6"
If-None-Match: "foobar","A34FAC0095","Profiles in Courage"
Weak and Strong Validators
Caches use entity tags to determine whether the cached version is up-to-date with
respect to the server (much like they use last-modified dates). In this way, entity tags
and last-modified dates both are cache validators.
Servers may sometimes want to allow cosmetic or insignificant changes to docu-
ments without invalidating all cached copies. HTTP/1.1 supports “weak validators,”
which allow the server to claim “good enough” equivalence even if the contents have
changed slightly.
Strong validators change any time the content changes. Weak validators allow some
content change but generally change when the significant meaning of the content
changes. Some operations cannot be performed using weak validators (such as condi-
tional partial-range fetches), so servers identify validators that are weak with a “W/”
prefix:
ETag: W/"v2.6"
If-None-Match: W/"v2.6"
A strong entity tag must change whenever the associated entity value changes in any
way. A weak entity tag should change whenever the associated entity changes in a
semantically significant way.
Note that an origin server must avoid reusing a specific strong entity tag value for two
different entities, or reusing a specific weak entity tag value for two semantically differ-
ent entities. Cache entries might persist for arbitrarily long periods, regardless of expi-
ration times, so it might be inappropriate to expect that a cache will never again
attempt to validate an entry using a validator that it obtained at some point in the past.
When to Use Entity Tags and Last-Modified Dates
HTTP/1.1 clients must use an entity tag validator if a server sends back an entity tag. If
the server sends back only a Last-Modified value, the client can use If-Modified-Since
validation. If both an entity tag and a last-modified date are available, the client should
use both revalidation schemes, allowing both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
182 |Chapter 7: Caching
HTTP/1.1 origin servers should send an entity tag validator unless it is not feasible to
generate one, and it may be a weak entity tag instead of a strong entity tag, if there
are benefits to weak validators. Also, it’s preferred to also send a last-modified value.
If an HTTP/1.1 cache or server receives a request with both If-Modified-Since and
entity tag conditional headers, it must not return a 304 Not Modified response
unless doing so is consistent with all of the conditional header fields in the request.
Controlling Cachability
HTTP defines several ways for a server to specify how long a document can be
cached before it expires. In decreasing order of priority, the server can:
• Attach a Cache-Control: no-store header to the response.
• Attach a Cache-Control: no-cache header to the response.
• Attach a Cache-Control: must-revalidate header to the response.
• Attach a Cache-Control: max-age header to the response.
• Attach an Expires date header to the response.
• Attach no expiration information, letting the cache determine its own heuristic
expiration date.
This section describes the cache controlling headers. The next section, “Setting Cache
Controls,” describes how to assign different cache information to different content.
No-Cache and No-Store Response Headers
HTTP/1.1 offers several ways to limit the caching of objects, or the serving of cached
objects, to maintain freshness. The no-store and no-cache headers prevent caches
from serving unverified cached objects:
Cache-Control: no-store
Cache-Control: no-cache
Pragma: no-cache
A response that is marked "no-store" forbids a cache from making a copy of the
response. A cache would typically forward a no-store response to the client, and
then delete the object, as would a non-caching proxy server.
A response that is marked "no-cache" can actually be stored in the local cache stor-
age. It just cannot be served from the cache to the client without first revalidating
the freshness with the origin server. A better name for this header might be "do-not-
serve-from-cache-without-revalidation."
The Pragma: no-cache header is included in HTTP/1.1 for backward compatibility
with HTTP/1.0+. HTTP 1.1 applications should use Cache-Control: no-cache, except
when dealing with HTTP 1.0 applications, which understand only Pragma: no-cache.*
Controlling Cachability |183
Max-Age Response Headers
The Cache-Control: max-age header indicates the number of seconds since it came
from the server for which a document can be considered fresh. There is also an s-
maxage header (note the absence of a hyphen in “maxage”) that acts like max-age
but applies only to shared (public) caches:
Cache-Control: max-age=3600
Cache-Control: s-maxage=3600
Servers can request that caches either not cache a document or refresh on every
access by setting the maximum aging to zero:
Cache-Control: max-age=0
Cache-Control: s-maxage=0
Expires Response Headers
The deprecated Expires header specifies an actual expiration date instead of a time in sec-
onds. The HTTP designers later decided that, because many servers have unsynchro-
nized or incorrect clocks, it would be better to represent expiration in elapsed seconds,
rather than absolute time. An analogous freshness lifetime can be calculated by comput-
ing the number of seconds difference between the expires value and the date value:
Expires: Fri, 05 Jul 2002, 05:00:00 GMT
Some servers also send back an Expires: 0 response header to try to make docu-
ments always expire, but this syntax is illegal and can cause problems with some
software. You should try to support this construct as input, but shouldn’t generate it.
Must-Revalidate Response Headers
Caches may be configured to serve stale (expired) objects, in order to improve per-
formance. If an origin server wishes caches to strictly adhere to expiration informa-
tion, it can attach a Cache-Control:
Cache-Control: must-revalidate
The Cache-Control: must-revalidate response header tells caches they cannot serve a
stale copy of this object without first revalidating with the origin server. Caches are
still free to serve fresh copies. If the origin server is unavailable when a cache
attempts a must-revalidate freshness check, the cache must return a 504 Gateway
Timeout error.
* Pragma no-cache is technically valid only for HTTP requests, yet it is widely used as an extension header for
both HTTP erquests and responses.
184 |Chapter 7: Caching
Heuristic Expiration
If the response doesn’t contain either a Cache-Control: max-age header or an Expires
header, the cache may compute a heuristic maximum age. Any algorithm may be
used, but if the resulting maximum age is greater than 24 hours, a Heuristic Expira-
tion Warning (Warning 13) header should be added to the response headers. As far
as we know, few browsers make this warning information available to users.
One popular heuristic expiration algorithm, the LM-Factor algorithm, can be used if
the document contains a last-modified date. The LM-Factor algorithm uses the last-
modified date as an estimate of how volatile a document is. Here’s the logic:
• If a cached document was last changed in the distant past, it may be a stable
document and less likely to change suddenly, so it is safer to keep it in the cache
longer.
• If the cached document was modified just recently, it probably changes fre-
quently, so we should cache it only a short while before revalidating with the
server.
The actual LM-Factor algorithm computes the time between when the cache talked
to the server and when the server said the document was last modified, takes some
fraction of this intervening time, and uses this fraction as the freshness duration in
the cache. Here is some Perl pseudocode for the LM-factor algorithm:
$time_since_modify = max(0, $server_Date - $server_Last_Modified);
$server_freshness_limit = int($time_since_modify * $lm_factor);
Figure 7-16 depicts the LM-factor freshness period graphically. The cross-hatched
line indicates the freshness period, using an LM-factor of 0.2.
Typically, people place upper bounds on heuristic freshness periods so they can’t
grow excessively large. A week is typical, though more conservative sites use a day.
Finally, if you don’t have a last-modified date either, the cache doesn’t have much
information to go on. Caches typically assign a default freshness period (an hour or a
day is typical) for documents without any freshness clues. More conservative caches
sometimes choose freshness lifetimes of 0 for these heuristic documents, forcing the
cache to validate that the data is still fresh before each time it is served to a client.
Figure 7-16. Computing a freshness period using the LM-Factor algorithm
20% of time between fetch
and last modification
Cached copy is fresh for
time period New expiration time
Last modified When cache talked
to server
Time
(LM-factor= 0.2)
Controlling Cachability |185
One last note about heuristic freshness calculations—they are more common than
you might think. Many origin servers still don’t generate Expires and max-age head-
ers. Pick your cache’s expiration defaults carefully!
Client Freshness Constraints
Web browsers have a Refresh or Reload button to forcibly refresh content, which
might be stale in the browser or proxy caches. The Refresh button issues a GET
request with additional Cache-control request headers that force a revalidation or
unconditional fetch from the server. The precise Refresh behavior depends on the
particular browser, document, and intervening cache configurations.
Clients use Cache-Control request headers to tighten or loosen expiration con-
straints. Clients can use Cache-control headers to make the expiration more strict,
for applications that need the very freshest documents (such as the manual Refresh
button). On the other hand, clients might also want to relax the freshness require-
ments as a compromise to improve performance, reliability, or expenses. Table 7-4
summarizes the Cache-Control request directives.
Cautions
Document expiration isn’t a perfect system. If a publisher accidentally assigns an
expiration date too far in the future, any document changes she needs to make won’t
necessarily show up in all caches until the document has expired.*For this reason,
Table 7-4. Cache-Control request directives
Directive Purpose
Cache-Control: max-stale
Cache-Control: max-stale =
<s>
The cache is free to serve a stale document. If the <s> parameter is specified, the docu-
ment must not be stale by more than this amount of time. This relaxes the caching rules.
Cache-Control: min-fresh =
<s>
The document must still be fresh for at least <s> seconds in the future. This makes the
caching rules more strict.
Cache-Control: max-age = <s> The cache cannot return a document that has been cached for longer than <s> seconds.
This directive makes the caching rules more strict, unless the max-stale directive also is
set, in which case the age can exceed its expiration time.
Cache-Control: no-cache
Pragma: no-cache
This client won’t accept a cached resource unless it has been revalidated.
Cache-Control: no-store The cache should delete every trace of the document from storage as soon as possible,
because it might contain sensitive information.
Cache-Control: only-if-cached The client wants a copy only if it is in the cache.
* Document expiration is a form of “time to live” technique used in many Internet protocols, such as DNS. DNS,
like HTTP, has trouble if you publish an expiration date far in the future and then find that you need to make
a change. However, HTTP provides mechanisms for a client to override and force a reloading, unlike DNS.
186 |Chapter 7: Caching
many publishers don’t use distant expiration dates. Also, many publishers don’t even
use expiration dates, making it tough for caches to know how long the document
will be fresh.
Setting Cache Controls
Different web servers provide different mechanisms for setting HTTP cache-control
and expiration headers. In this section, we’ll talk briefly about how the popular
Apache web server supports cache controls. Refer to your web server documentation
for specific details.
Controlling HTTP Headers with Apache
The Apache web server provides several mechanisms for setting HTTP cache-
controlling headers. Many of these mechanisms are not enabled by default—you
have to enable them (in some cases first obtaining Apache extension modules). Here
is a brief description of some of the Apache features:
mod_headers
The mod_headers module lets you set individual headers. Once this module is
loaded, you can augment the Apache configuration files with directives to set
individual HTTP headers. You also can use these settings in combination with
Apache’s regular expressions and filters to associate headers with individual con-
tent. Here is an example of a configuration that could mark all HTML files in a
directory as uncachable:
<Files *.html>
Header set Cache-control no-cache
</Files>
mod_expires
The mod_expires module provides program logic to automatically generate
Expires headers with the correct expiration dates. This module allows you to set
expiration dates for some time period after a document was last accessed or after
its last-modified date. The module also lets you assign different expiration dates
to different file types and use convenient verbose descriptions, like “access plus 1
month,” to describe cachability. Here are a few examples:
ExpiresDefault A3600
ExpiresDefault M86400
ExpiresDefault "access plus 1 week"
ExpiresByType text/html "modification plus 2 days 6 hours 12 minutes"
mod_cern_meta
The mod_cern_meta module allows you to associate a file of HTTP headers with
particular objects. When you enable this module, you create a set of “metafiles,”
one for each document you want to control, and add the desired headers to each
metafile.
Detailed Algorithms |187
Controlling HTML Caching Through HTTP-EQUIV
HTTP server response headers are used to carry back document expiration and
cache-control information. Web servers interact with configuration files to assign the
correct cache-control headers to served documents.
To make it easier for authors to assign HTTP header information to served HTML
documents without interacting with web server configuration files, HTML 2.0
defined the <META HTTP-EQUIV> tag. This optional tag sits at the top of an
HTML document and defines HTTP headers that should be associated with the doc-
ument. Here is an example of a <META HTTP-EQUIV> tag set to mark the HTML
document uncachable:
<HTML>
<HEAD>
<TITLE>My Document</TITLE>
<META HTTP-EQUIV="Cache-control" CONTENT="no-cache">
</HEAD>
...
This HTTP-EQUIV tag was originally intended to be used by web servers. Web serv-
ers were supposed to parse HTML for <META HTTP-EQUIV> tags and insert the
prescribed headers into the HTTP response, as documented in HTML RFC 1866:
An HTTP server may use this information to process the document. In particular, it
may include a header field in the responses to requests for this document: the header
name is taken from the HTTP-EQUIV attribute value, and the header value is taken
from the value of the CONTENT attribute.
Unfortunately, few web servers and proxies support this optional feature because of
the extra server load, the values being static, and the fact that it supports only HTML
and not the many other file types.
However, some browsers do parse and adhere to HTTP-EQUIV tags in the HTML
content, treating the embedded headers like real HTTP headers (Figure 7-17). This is
unfortunate, because HTML browsers that do support HTTP-EQUIV may apply dif-
ferent cache-control rules than intervening proxy caches. This causes confusing
cache expiration behavior.
In general, <META HTTP-EQUIV> tags are a poor way of controlling document
cachability. The only sure-fire way to communicate cache-control requests for docu-
ments is through HTTP headers sent by a properly configured server.
Detailed Algorithms
The HTTP specification provides a detailed, but slightly obscure and often confus-
ing, algorithm for computing document aging and cache freshness. In this section,
we’ll discuss the HTTP freshness computation algorithms in detail (the “Fresh
enough?” diamond in Figure 7-12) and explain the motivation behind them.
188 |Chapter 7: Caching
This section will be most useful to readers working with cache internals. To help
illustrate the wording in the HTTP specification, we will make use of Perl
pseudocode. If you aren’t interested in the gory details of cache expiration formulas,
feel free to skip this section.
Age and Freshness Lifetime
To tell whether a cached document is fresh enough to serve, a cache needs to com-
pute only two values: the cached copy’s age and the cached copy’s freshness lifetime.
If the age of a cached copy is less than the freshness lifetime, the copy is fresh enough
to serve. In Perl:
$is_fresh_enough = ($age < $freshness_lifetime);
Figure 7-17. HTTP-EQUIV tags cause problems, because most software ignores them
Some HTTP servers can be configured to parse HTML files for special
<META HTTP-EQUIV> tags. These metadata tags (in the HTML document)
describe HTTP headers that the author would like to be received by the client.
Unfortunately, most web servers don’t process HTTP-EQUIV tags, and even
fewer proxies do. This causes client caches to receive cache-control commands
that proxy caches do not always see.
Client Server
GET /xyz.html HTTP/1.0
HTTP request
HTTP/1.0 200 OK
Date: Fri, 07 Apr 2002, 19:21:13 GMT
Content-length: 124
Cache-control: max-age=3600
Content-type: text/html; charset=utf-8
<HTML>
<HEAD>
<META HTTP-EQUIV="Cache-control"
CONTENT="max-age=3600"
<META HTTP-EQUIV="Content-type"
CONTENT="text/html; charset=utf-8"
</HEAD>
<BODY>
Welcome to XYZ Industries, a <B>leader</B>
in mechanical drilling machines for...
HTTP response
<HTML>
<HEAD>
<META HTTP-EQUIV="Cache-control"
CONTENT="max-age=3600">
<META HTTP-EQUIV="Content-type"
CONTENT="text/html; charset=utf-8">
</HEAD>
<BODY>
Welcome to XYZ Industries, a
<B>leader</B> in mechanical drilling
machines for 30 years. Our new line of
100% automated manufacturing tools sets
the standard for CAM, at a suprisingly
low price.
</BODY>
HTML file
Some servers will insert HTTP-EQUIV specified headers into
the response header for proxies to see. Others servers will not.
Detailed Algorithms |189
The age of the document is the total time the document has “aged” since it was sent
from the server (or was last revalidated by the server).*Because a cache might not
know if a document response is coming from an upstream cache or a server, it can’t
assume that the document is brand new. It must determine the document’s age,
either from an explicit Age header (preferred) or by processing the server-generated
Date header.
The freshness lifetime of a document tells how old a cached copy can get before it is
no longer fresh enough to serve to clients. The freshness lifetime takes into account the
expiration date of the document and any freshness overrides the client might request.
Some clients may be willing to accept slightly stale documents (using the Cache-Con-
trol: max-stale header). Other clients may not accept documents that will become
stale in the near future (using the Cache-Control: min-fresh header). The cache com-
bines the server expiration information with the client freshness requirements to
determine the maximum freshness lifetime.
Age Computation
The age of the response is the total time since the response was issued from the
server (or revalidated from the server). The age includes the time the response has
floated around in the routers and gateways of the Internet, the time stored in inter-
mediate caches, and the time the response has been resident in your cache.
Example 7-1 provides pseudocode for the age calculation.
The particulars of HTTP age calculation are a bit tricky, but the basic concept is sim-
ple. Caches can tell how old the response was when it arrived at the cache by exam-
ining the Date or Age headers. Caches also can note how long the document has
been sitting in the local cache. Summed together, these values are the entire age of
the response. HTTP throws in some magic to attempt to compensate for clock skew
and network delays, but the basic computation is simple enough:
$age = $age_when_document_arrived_at_our_cache +
$how_long_copy_has_been_in_our_cache;
* Remember that the server always has the most up-to-date version of any document.
Example 7-1. HTTP/1.1 age-calculation algorithm calculates the overall age of a cached document
$apparent_age = max(0, $time_got_response - $Date_header_value);
$corrected_apparent_age = max($apparent_age, $Age_header_value);
$response_delay_estimate = ($time_got_response - $time_issued_request);
$age_when_document_arrived_at_our_cache =
$corrected_apparent_age + $response_delay_estimate;
$how_long_copy_has_been_in_our_cache = $current_time - $time_got_response;
$age = $age_when_document_arrived_at_our_cache +
$how_long_copy_has_been_in_our_cache;
190 |Chapter 7: Caching
A cache can pretty easily determine how long a cached copy has been cached locally
(a matter of simple bookkeeping), but it is harder to determine the age of a response
when it arrives at the cache, because not all servers have synchronized clocks and
because we don’t know where the response has been. The complete age-calculation
algorithm tries to remedy this.
Apparent age is based on the Date header
If all computers shared the same, exactly correct clock, the age of a cached document
would simply be the “apparent age” of the document—the current time minus the
time when the server sent the document. The server send time is simply the value of
the Date header. The simplest initial age calculation would just use the apparent age:
$apparent_age = $time_got_response - $Date_header_value;
$age_when_document_arrived_at_our_cache = $apparent_age;
Unfortunately, not all clocks are well synchronized. The client and server clocks may
differ by many minutes, or even by hours or days when clocks are set improperly.*
Web applications, especially caching proxies, have to be prepared to interact with
servers with wildly differing clock values. The problem is called clock skew—the dif-
ference between two computers’ clock settings. Because of clock skew, the apparent
age sometimes is inaccurate and occasionally is negative.
If the age is ever negative, we just set it to zero. We also could sanity check that the
apparent age isn’t ridiculously large, but large apparent ages might actually be cor-
rect. We might be talking to a parent cache that has cached the document for a long
time (the cache also stores the original Date header):
$apparent_age = max(0, $time_got_response - $Date_header_value);
$age_when_document_arrived_at_our_cache = $apparent_age;
Be aware that the Date header describes the original origin server date. Proxies and
caches must not change this date!
Hop-by-hop age calculations
So, we can eliminate negative ages caused by clock skew, but we can’t do much
about overall loss of accuracy due to clock skew. HTTP/1.1 attempts to work around
the lack of universal synchronized clocks by asking each device to accumulate rela-
tive aging into an Age header, as a document passes through proxies and caches.
This way, no cross-server, end-to-end clock comparisons are needed.
The Age header value increases as the document passes through proxies. HTTP/1.1-
aware applications should augment the Age header value by the time the document
* The HTTP specification recommends that clients, servers, and proxies use a time synchronization protocol
such as NTP to enforce a consistent time base.
Detailed Algorithms |191
sat in each application and in network transit. Each intermediate application can eas-
ily compute the document’s resident time by using its local clock.
However, any non-HTTP/1.1 device in the response chain will not recognize the Age
header and will either proxy the header unchanged or remove it. So, until HTTP/1.1
is universally adopted, the Age header will be an underestimate of the relative age.
The relative age values are used in addition to the Date-based age calculation, and
the most conservative of the two age estimates is chosen, because either the cross-
server Date value or the Age-computed value may be an underestimate (the most
conservative is the oldest age). This way, HTTP tolerates errors in Age headers as
well, while erring on the side of fresher content:
$apparent_age = max(0, $time_got_response - $Date_header_value);
$corrected_apparent_age = max($apparent_age, $Age_header_value);
$age_when_document_arrived_at_our_cache = $corrected_apparent_age;
Compensating for network delays
Transactions can be slow. This is the major motivation for caching. But for very slow
networks, or overloaded servers, the relative age calculation may significantly under-
estimate the age of documents if the documents spend a long time stuck in network
or server traffic jams.
The Date header indicates when the document left the origin server,*but it doesn’t
say how long the document spent in transit on the way to the cache. If the docu-
ment came through a long chain of proxies and parent caches, the network delay
might be significant.†
There is no easy way to measure one-way network delay from server to cache, but it
is easier to measure the round-trip delay. A cache knows when it requested the docu-
ment and when it arrived. HTTP/1.1 conservatively corrects for these network delays
by adding the entire round-trip delay. This cache-to-server-to-cache delay is an over-
estimate of the server-to-cache delay, but it is conservative. If it is in error, it will only
make the documents appear older than they really are and cause unnecessary revali-
dations. Here’s how the calculation is made:
$apparent_age = max(0, $time_got_response - $Date_header_value);
$corrected_apparent_age = max($apparent_age, $Age_header_value);
$response_delay_estimate = ($time_got_response - $time_issued_request);
$age_when_document_arrived_at_our_cache =
$corrected_apparent_age + $response_delay_estimate;
* Note that if the document came from a parent cache and not from an origin server, the Date header will
reflect the date of the origin server, not of the parent cache.
† In practice, this shouldn’t be more than a few tens of seconds (or users will abort), but the HTTP designers
wanted to try to support accurate expiration of even of short-lifetime objects.
192 |Chapter 7: Caching
Complete Age-Calculation Algorithm
The last section showed how to compute the age of an HTTP-carried document when
it arrives at a cache. Once this response is stored in the cache, it ages further. When a
request arrives for the document in the cache, we need to know how long the docu-
ment has been resident in the cache, so we can compute the current document age:
$age = $age_when_document_arrived_at_our_cache +
$how_long_copy_has_been_in_our_cache;
Ta-da! This gives us the complete HTTP/1.1 age-calculation algorithm we presented
in Example 7-1. This is a matter of simple bookkeeping—we know when the docu-
ment arrived at the cache ($time_got_response) and we know when the current
request arrived (right now), so the resident time is just the difference. This is all
shown graphically in Figure 7-18.
Freshness Lifetime Computation
Recall that we’re trying to figure out whether a cached document is fresh enough to
serve to a client. To answer this question, we must determine the age of the cached
document and compute the freshness lifetime based on server and client constraints.
We just explained how to compute the age; now let’s move on to freshness lifetimes.
The freshness lifetime of a document tells how old a document is allowed to get before
it is no longer fresh enough to serve to a particular client. The freshness lifetime
Figure 7-18. The age of a cached document includes resident time in the network and cache
Server
Client
Cache
Server processing
time Server processing
time Response’s
network delay
time_issued_request
date_value
time_got_response
current_time
time_client_issued_request
cache resident time
Age of cached document
Detailed Algorithms |193
depends on server and client constraints. The server may have information about the
publication change rate of the document. Very stable, filed reports may stay fresh for
years. Periodicals may be up-to-date only for the time remaining until the next sched-
uled publication—next week, or 6:00 am tomorrow.
Clients may have certain other guidelines. They may be willing to accept slightly
stale content, if it is faster, or they might need the most up-to-date content possible.
Caches serve the users. We must adhere to their requests.
Complete Server-Freshness Algorithm
Example 7-2 shows a Perl algorithm to compute server freshness limits. It returns the
maximum age that a document can reach and still be served by the server.
Example 7-2. Server freshness constraint calculation
sub server_freshness_limit
{
local($heuristic,$server_freshness_limit,$time_since_last_modify);
$heuristic = 0;
if ($Max_Age_value_set)
{
$server_freshness_limit = $Max_Age_value;
}
elsif ($Expires_value_set)
{
$server_freshness_limit = $Expires_value - $Date_value;
}
elsif ($Last_Modified_value_set)
{
$time_since_last_modify = max(0, $Date_value - $Last_Modified_value);
$server_freshness_limit = int($time_since_last_modify * $lm_factor);
$heuristic = 1;
}
else
{
$server_freshness_limit = $default_cache_min_lifetime;
$heuristic = 1;
}
if ($heuristic)
{
if ($server_freshness_limit > $default_cache_max_lifetime)
{ $server_freshness_limit = $default_cache_max_lifetime; }
if ($server_freshness_limit < $default_cache_min_lifetime)
{ $server_freshness_limit = $default_cache_min_lifetime; }
}
return($server_freshness_limit);
}
194 |Chapter 7: Caching
Now let’s look at how the client can override the document’s server-specified age
limit. Example 7-3 shows a Perl algorithm to take a server freshness limit and mod-
ify it by the client constraints. It returns the maximum age that a document can
reach and still be served by the cache without revalidation.
The whole process involves two variables: the document’s age and its freshness limit.
The document is “fresh enough” if the age is less than the freshness limit. The algo-
rithm in Example 7-3 just takes the server freshness limit and slides it around based
on additional client constraints. We hope this section made the subtle expiration
algorithms described in the HTTP specifications a bit clearer.
Caches and Advertising
If you’ve made it this far, you’ve realized that caches improve performance and
reduce traffic. You know caches can help users and give them a better experience,
and you know caches can help network operators reduce their traffic.
The Advertiser’s Dilemma
You might also expect content providers to like caches. After all, if caches were
everywhere, content providers wouldn’t have to buy big multiprocessor web servers
to keep up with demand—and they wouldn’t have to pay steep network service
charges to feed the same data to their viewers over and over again. And better yet,
Example 7-3. Client freshness constraint calculation
sub client_modified_freshness_limit
{
$age_limit = server_freshness_limit( ); ## From Example 7-2
if ($Max_Stale_value_set)
{
if ($Max_Stale_value == $INT_MAX)
{ $age_limit = $INT_MAX; }
else
{ $age_limit = server_freshness_limit( ) + $Max_Stale_value; }
}
if ($Min_Fresh_value_set)
{
$age_limit = min($age_limit, server_freshness_limit( ) - $Min_Fresh_value_set);
}
if ($Max_Age_value_set)
{
$age_limit = min($age_limit, $Max_Age_value);
}
}
Caches and Advertising |195
caches make the flashy articles and advertisements show up even faster and look
even better on the viewer’s screens, encouraging them to consume more content and
see more advertisements. And that’s just what content providers want! More eye-
balls and more advertisements!
But that’s the rub. Many content providers are paid through advertising—in particu-
lar, they get paid every time an advertisement is shown to a user (maybe just a frac-
tion of a penny or two, but they add up if you show a million ads a day!). And that’s
the problem with caches—they can hide the real access counts from the origin
server. If caching was perfect, an origin server might not receive any HTTP accesses
at all, because they would be absorbed by Internet caches. But, if you are paid on
access counts, you won’t be celebrating.
The Publisher’s Response
Today, advertisers use all sorts of “cache-busting” techniques to ensure that caches
don’t steal their hit stream. They slap no-cache headers on their content. They serve
advertisements through CGI gateways. They rewrite advertisement URLs on each
access.
And these cache-busting techniques aren’t just for proxy caches. In fact, today they
are targeted primarily at the cache that’s enabled in every web browser. Unfortu-
nately, while over-aggressively trying to maintain their hit stream, some content pro-
viders are reducing the positive effects of caching to their site.
In the ideal world, content providers would let caches absorb their traffic, and the
caches would tell them how many hits they got. Today, there are a few ways caches
can do this.
One solution is to configure caches to revalidate with the origin server on every
access. This pushes a hit to the origin server for each access but usually does not
transfer any body data. Of course, this slows down the transaction.*
Log Migration
One ideal solution wouldn’t require sending hits through to the server. After all, the
cache can keep a log of all the hits. Caches could just distribute the hit logs to serv-
ers. In fact, some large cache providers have been know to manually process and
hand-deliver cache logs to influential content providers to keep the content provid-
ers happy.
* Some caches support a variant of this revalidation, where they do a conditional GET or a HEAD request in
the background. The user does not perceive the delay, but the request triggers an offline access to the origin
server. This is an improvement, but it places more load on the caches and significantly increases traffic across
the network.
196 |Chapter 7: Caching
Unfortunately, hit logs are large, which makes them tough to move. And cache logs
are not standardized or organized to separate logs out to individual content provid-
ers. Also, there are authentication and privacy issues.
Proposals have been made for efficient (and less efficient) log-redistribution schemes.
None are far enough developed to be adopted by web software vendors. Many are
extremely complex and require joint business partnerships to succeed.*Several cor-
porate ventures have been launched to develop supporting infrastructure for adver-
tising revenue reclamation.
Hit Metering and Usage Limiting
RFC 2227, “Simple Hit-Metering and Usage-Limiting for HTTP,” defines a much sim-
pler scheme. This protocol adds one new header to HTTP, called Meter, that periodi-
cally carries hit counts for particular URLs back to the servers. This way, servers get
periodic updates from caches about the number of times cached documents were hit.
In addition, the server can control how many times documents can be served from
cache, or a wall clock timeout, before the cache must report back to the server. This
is called usage limiting; it allows servers to control how much a cached resource can
be used before it needs to report back to the origin server.
We’ll describe RFC 2227 in detail in Chapter 21.
For More Information
For more information on caching, refer to:
http://www.w3.org/Protocols/rfc2616/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol,” by R. Fielding, J. Gettys, J. Mogul, H.
Frystyk, L. Mastinter, P. Leach, and T. Berners-Lee.
Web Caching
Duane Wessels, O’Reilly & Associates, Inc.
http://search.ietf.org/rfc/rfc3040.txt
RFC 3040, “Internet Web Replication and Caching Taxonomy.”
Web Proxy Servers
Ari Luotonen, Prentice Hall Computer Books.
http://search.ietf.org/rfc/rfc3143.txt
RFC 3143, “Known HTTP Proxy/Caching Problems.”
http://www.squid-cache.org
Squid Web Proxy Cache.
* Several businesses have launched trying to develop global solutions for integrated caching and logging.
197
CHAPTER 8
Integration Points: Gateways,
Tunnels, and Relays
The Web has proven to be an incredible tool for disseminating content. Over time,
people have moved from just wanting to put static documents online to wanting to
share ever more complex resources, such as database content or dynamically gener-
ated HTML pages. HTTP applications, like web browsers, have provided users with
a unified means of accessing content over the Internet.
HTTP also has come to be a fundamental building block for application developers,
who piggyback other protocols on top of HTTP (for example, using HTTP to tunnel
or relay other protocol traffic through corporate firewalls, by wrapping that traffic in
HTTP). HTTP is used as a protocol for all of the Web’s resources, and it’s also a pro-
tocol that other applications and application protocols make use of to get their jobs
done.
This chapter takes a general look at some of the methods that developers have
come up with for using HTTP to access different resources and examines how
developers use HTTP as a framework for enabling other protocols and application
communication.
In this chapter, we discuss:
• Gateways, which interface HTTP with other protocols and applications
• Application interfaces, which allow different types of web applications to com-
municate with one another
• Tunnels, which let you send non-HTTP traffic over HTTP connections
• Relays, which are a type of simplified HTTP proxy used to forward data one hop
at a time
Gateways
The history behind HTTP extensions and interfaces was driven by people’s needs.
When the desire to put more complicated resources on the Web emerged, it rapidly
became clear that no single application could handle all imaginable resources.
198 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
To get around this problem, developers came up with the notion of a gateway that
could serve as a sort of interpreter, abstracting a way to get at the resource. A gate-
way is the glue between resources and applications. An application can ask (through
HTTP or some other defined interface) a gateway to handle the request, and the
gateway can provide a response. The gateway can speak the query language to the
database or generate the dynamic content, acting like a portal: a request goes in, and
a response comes out.
Figure 8-1 depicts a kind of resource gateway. Here, the Joe’s Hardware server is act-
ing as a gateway to database content—note that the client is simply asking for a
resource through HTTP, and the Joe’s Hardware server is interfacing with a gateway
to get at the resource.
Some gateways automatically translate HTTP traffic to other protocols, so HTTP cli-
ents can interface with other applications without the clients needing to know other
protocols (Figure 8-2).
Figure 8-2 shows three examples of gateways:
• In Figure 8-2a, the gateway receives HTTP requests for FTP URLs. The gateway
then opens FTP connections and issues the appropriate commands to the FTP
server. The document is sent back through HTTP, along with the correct HTTP
headers.
• In Figure 8-2b, the gateway receives an encrypted web request through SSL,
decrypts the request,*and forwards a normal HTTP request to the destination
server. These security accelerators can be placed directly in front of web servers
(usually in the same premises) to provide high-performance encryption for ori-
gin servers.
Figure 8-1. Gateway magic
* The gateway would need to have the proper server certificates installed.
Client is requesting:
http://www.joes-hardware.com/query-db.cgi?newproducts
GET /query-db.cgi?newproducts HTTP/1.1
Host: www.joes-hardware.com
Accept: *
Client
www.joes-hardware.com
HTTP/1.0 200 OK
New product list:
... Gateway Database
Request message
Response message
Gateways |199
• In Figure 8-2c, the gateway connects HTTP clients to server-side application
programs, through an application server gateway API. When you purchase from
e-commerce stores on the Web, check the weather forecast, or get stock quotes,
you are visiting application server gateways.
Client-Side and Server-Side Gateways
Web gateways speak HTTP on one side and a different protocol on the other side.*
Gateways are described by their client- and server-side protocols, separated by a slash:
<client-protocol>/<server-protocol>
So a gateway joining HTTP clients to NNTP news servers is an HTTP/NNTP gate-
way. We use the terms “server-side gateway” and “client-side gateway” to describe
what side of the gateway the conversion is done for:
•Server-side gateways speak HTTP with clients and a foreign protocol with serv-
ers (HTTP/*).
•Client-side gateways speak foreign protocols with clients and HTTP with servers
(*/HTTP).
Figure 8-2. Three web gateway examples
* Web proxies that convert between different versions of HTTP are like gateways, because they perform
sophisticated logic to negotiate between the parties. But because they speak HTTP on both sides, they are
technically proxies.
HTTP client FTP serverGateway
HTTP FTP
(a) HTTP/FTP server-side FTP gateway
HTTPS client Web serverGateway
SSL HTTP
(b) HTTPS/HTTP client-side security gateway
HTTP client Application server gateway
HTTP
(c) HTTP/CGI server-side application gateway
CGI (or other API)
App server Program
200 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
Protocol Gateways
You can direct HTTP traffic to gateways the same way you direct traffic to proxies.
Most commonly, you explicitly configure browsers to use gateways, intercept traffic
transparently, or configure gateways as surrogates (reverse proxies).
Figure 8-3 shows the dialog boxes used to configure a browser to use server-side FTP
gateways. In the configuration shown, the browser is configured to use gw1.joes-
hardware.com as an HTTP/FTP gateway for all FTP URLs. Instead of sending FTP
commands to an FTP server, the browser will send HTTP commands to the HTTP/
FTP gateway gw1.joes-hardware.com on port 8080.
The result of this gateway configuration is shown in Figure 8-4. Normal HTTP traf-
fic is unaffected; it continues to flow directly to origin servers. But requests for FTP
URLs are sent to the gateway gw1.joes-hardware.com within HTTP requests. The
gateway performs the FTP transactions on the client’s behalf and carries results back
to the client by HTTP.
The following sections describe common kinds of gateways: server protocol con-
verters, server-side security gateways, client-side security gateways, and application
servers.
HTTP/*: Server-Side Web Gateways
Server-side web gateways convert client-side HTTP requests into a foreign protocol,
as the requests travel inbound to the origin server (see Figure 8-5).
In Figure 8-5, the gateway receives an HTTP request for an FTP resource:
ftp://ftp.irs.gov/pub/00-index.txt
Figure 8-3. Configuring an HTTP/FTP gateway
(a) MSIE manual proxy settings (b) Navigator manual proxy settings
Protocol Gateways |201
The gateway proceeds to open an FTP connection to the FTP port on the origin
server (port 21) and speak the FTP protocol to fetch the object. The gateway does
the following:
• Sends the USER and PASS commands to log in to the server
• Issues the CWD command to change to the proper directory on the server
• Sets the download type to ASCII
• Fetches the document’s last modification time with MDTM
• Tells the server to expect a passive data retrieval request using PASV
• Requests the object retrieval using RETR
• Opens a data connection to the FTP server on a port returned on the control
channel; as soon as the data channel is opened, the object content flows back to
the gateway
Figure 8-4. Browsers can configure particular protocols to use particular gateways
Figure 8-5. The HTTP/FTP gateway translates HTTP request into FTP requests
HTTP client
Web server
(www.cnn.com)
GET http://www.cnn.com/HTTP/1.0
Host: www.cnn.com
User-agent: SuperBrowser 4.2
HTTP
FTP server
(ftp.irs.gov)
GET ftp://ftp.irs.gov/pub/00-index.txt HTTP/1.0
Host: ftp.irs.gov
User-agent: SuperBrowser 4.2
HTTP
HTTP/FTP gateway
(gw1.joes-hardware.com)
FTP
8080
HTTP client
HTTP
FTP server
GET ftp://ftp.irs.gov/pub/00-index.txt HTTP/1.0
Host: ftp.irs.gov
User-agent: SuperBrowser 4.2
HTTP/FTP inbound
conversion gateway
FTP control connection
Port 21
USER anonymous
PASS joe
CWD /pub
TYPE A
MDTM 00-index.txt
PASV
RETR 00-index.txt
FTP data connection
...data..
Inbound
202 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
When the retrieval is complete, the object will be sent to the client in an HTTP
response.
HTTP/HTTPS: Server-Side Security Gateways
Gateways can be used to provide extra privacy and security for an organization, by
encrypting all inbound web requests. Clients can browse the Web using normal
HTTP, but the gateway will automatically encrypt the user’s sessions (Figure 8-6).
HTTPS/HTTP: Client-Side Security Accelerator Gateways
Recently, HTTPS/HTTP gateways have become popular as security accelerators.
These HTTPS/HTTP gateways sit in front of the web server, usually as an invisible
intercepting gateway or a reverse proxy. They receive secure HTTPS traffic, decrypt
the secure traffic, and make normal HTTP requests to the web server (Figure 8-7).
These gateways often include special decryption hardware to decrypt secure traffic
much more efficiently than the origin server, removing load from the origin server.
Because these gateways send unencrypted traffic between the gateway and origin
server, you need to use caution to make sure the network between the gateway and
origin server is secure.
Figure 8-6. Inbound HTTP/HTTPS security gateway
Figure 8-7. HTTPS/HTTP security accelerator gateway
HTTP client
HTTP
Secure web
server
GET http://www.cnn.com/ HTTP/1.0
Host: www.cnn.com
User-agent: Superbrowser 4.2
HTTP/HTTPS inbound
security gateway
Port 443
mdsnrt734tngfd/p0f92piub5.
lod9fuo8w34b4/;p-90[g9yk,8
U|t6y6/%$!&9890G&*&98...
HTTP over SSL (HTTPS)
Browser www.cnn.com
GET http://www.cnn.com/ HTTP/1.0
Host: www.cnn.com
User-agent: Superbrowser 4.2
HTTPS/HTTP security
accelerator gateway
mdsnrt734tngfd/p0f92piub5.
lod9fuo8w34b4/;p-90[g9yk,8
U|t6y6/%$!&9890G&*&98...
HTTP over SSL (HTTPS) HTTP
Protected internal LAN
Resource Gateways |203
Resource Gateways
So far, we’ve been talking about gateways that connect clients and servers across a
network. However, the most common form of gateway, the application server, com-
bines the destination server and gateway into a single server. Application servers are
server-side gateways that speak HTTP with the client and connect to an application
program on the server side (see Figure 8-8).
In Figure 8-8, two clients are connecting to an application server using HTTP. But,
instead of sending back files from the server, the application server passes the
requests through a gateway application programming interface (API) to applications
running on the server:
• Client A’s request is received and, based on the URI, is sent through an API to a
digital camera application. The resulting camera image is bundled up into an
HTTP response message and sent back to the client, for display in the client’s
browser.
• Client B’s URI is for an e-commerce application. Client B’s requests are sent
through the server gateway API to the e-commerce software, and the results are
sent back to the browser. The e-commerce software interacts with the client,
walking the user through a sequence of HTML pages to complete a purchase.
The first popular API for application gateways was the Common Gateway Interface
(CGI). CGI is a standardized set of interfaces that web servers use to launch pro-
grams in response to HTTP requests for special URLs, collect the program output,
and send it back in HTTP responses. Over the past several years, commercial web
servers have provided more sophisticated interfaces for connecting web servers to
applications.
Figure 8-8. An application server connects HTTP clients to arbitrary backend applications
11000101101
Client A HTTP
Web camera API
E-commerce API
Client B
Application server
Camera device and software
E-commerce application
HTTP
204 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
Early web servers were fairly simple creations, and the simple approach that was
taken for implementing an interface for gateways has stuck to this day.
When a request comes in for a resource that needs a gateway, the server spawns the
helper application to handle the request. The helper application is passed the data it
needs. Often this is just the entire request or something like the query the user wants
to run on the database (from the query string of the URL; see Chapter 2).
It then returns a response or response data to the server, which vectors it off to the
client. The server and gateway are separate applications, so the lines of responsibil-
ity are kept clear. Figure 8-9 shows the basic mechanics behind server and gateway
application interactions.
This simple protocol (request in, hand off, and respond) is the essence behind the
oldest and one of the most common server extension interfaces, CGI.
Common Gateway Interface (CGI)
The Common Gateway Interface was the first and probably still is the most widely
used server extension. It is used throughout the Web for things like dynamic HTML,
credit card processing, and querying databases.
Since CGI applications are separate from the server, they can be implemented in
almost any language, including Perl, Tcl, C, and various shell languages. And
because CGI is simple, almost all HTTP servers support it. The basic mechanics of
the CGI model are shown in Figure 8-9.
CGI processing is invisible to users. From the perspective of the client, it’s just mak-
ing a normal request. It is completely unaware of the hand-off procedure going on
between the server and the CGI application. The client’s only hint that a CGI appli-
cation might be involved would be the presence of the letters “cgi” and maybe “?” in
the URL.
Figure 8-9. Server gateway application mechanics
Server system
Request 1
Request 2
Request N
Server process
Spawned gateway process #1
Spawned gateway process #2
Spawned gateway process #N
Response N
Response 2
Response 1
Request data
Response data
Server internal view
Application Interfaces and Web Services |205
So CGI is wonderful, right? Well, yes and no. It provides a simple, functional form of
glue between servers and pretty much any type of resource, handling any translation
that needs to occur. The interface also is elegant in protecting the server from buggy
extensions (if the extension were glommed onto the server itself, it could cause an
error that might end up crashing the server).
However, this separation incurs a cost in performance. The overhead to spawn a new
process for every CGI request is quite high, limiting the performance of servers that
use CGI and taxing the server machine’s resources. To try to get around this prob-
lem, a new form of CGI—aptly dubbed Fast CGI—has been developed. This inter-
face mimics CGI, but it runs as a persistent daemon, eliminating the performance
penalty of setting up and tearing down a new process for each request.
Server Extension APIs
The CGI protocol provides a clean way to interface external interpreters with stock
HTTP servers, but what if you want to alter the behavior of the server itself, or you just
want to eke every last drop of performance you can get out of your server? For these
two needs, server developers have provided server extension APIs, which provide a
powerful interface for web developers to interface their own modules with an HTTP
server directly. Extension APIs allow programmers to graft their own code onto the
server or completely swap out a component of the server and replace it with their own.
Most popular servers provide one or more extension APIs for developers. Since these
extensions often are tied to the architecture of the server itself, most of them are spe-
cific to one server type. Microsoft, Netscape, Apache, and other server flavors all
have API interfaces that allow developers to alter the behavior of the server or pro-
vide custom interfaces to different resources. These custom interfaces provide a pow-
erful interface for developers.
One example of a server extension is Microsoft’s FrontPage Server Extension (FPSE),
which supports web publishing services for FrontPage authors. FPSE is able to inter-
pret remote procedure call (RPC) commands sent by FrontPage clients. These com-
mands are piggybacked on HTTP (specifically, overlaid on the HTTP POST method).
For details, see “FrontPage Server Extensions for Publishing Support” in Chapter 19.
Application Interfaces and Web Services
We’ve discussed resource gateways as ways for web servers to communicate with
applications. More generally, with web applications providing ever more types of ser-
vices, it becomes clear that HTTP can be part of a foundation for linking together
applications. One of the trickier issues in wiring up applications is negotiating the
protocol interface between the two applications so that they can exchange data—
often this is done on an application-by-application basis.
206 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
To work together, applications usually need to exchange more complex information
with one another than is expressible in HTTP headers. A couple of examples of
extending HTTP or layering protocols on top of HTTP in order to exchange custom-
ized information are described in Chapter 19. “FrontPage Server Extensions for Pub-
lishing Support” in Chapter 19 talks about layering RPCs over HTTP POST
messages, and “WebDAV and Collaborative Authoring” talks about adding XML to
HTTP headers.
The Internet community has developed a set of standards and protocols that allow
web applications to talk to each other. These standards are loosely referred to as web
services, although the term can mean standalone web applications (building blocks)
themselves. The premise of web services is not new, but they are a new mechanism
for applications to share information. Web services are built on standard web tech-
nologies, such as HTTP.
Web services exchange information using XML over SOAP. The Extensible Markup
Language (XML) provides a way to create and interpret customized information
about a data object. The Simple Object Access Protocol (SOAP) is a standard for
adding XML information to HTTP messages.*
Tunnels
We’ve discussed different ways that HTTP can be used to enable access to various
kinds of resources (through gateways) and to enable application-to-application com-
munication. In this section, we’ll take a look at another use of HTTP, web tunnels,
which enable access to applications that speak non-HTTP protocols through HTTP
applications.
Web tunnels let you send non-HTTP traffic through HTTP connections, allowing
other protocols to piggyback on top of HTTP. The most common reason to use web
tunnels is to embed non-HTTP traffic inside an HTTP connection, so it can be sent
through firewalls that allow only web traffic.
Establishing HTTP Tunnels with CONNECT
Web tunnels are established using HTTP’s CONNECT method. The CONNECT pro-
tocol is not part of the core HTTP/1.1 specification,†but it is a widely implemented
extension. Technical specifications can be found in Ari Luotonen’s expired Internet
draft specification, “Tunneling TCP based protocols through Web proxy servers,” or
in his book Web Proxy Servers, both of which are cited at the end of this chapter.
* For more information, see http://www.w3.org/TR/2001/WD-soap12-part0-20011217/.Programming Web
Services with SOAP, by Doug Tidwell, James Snell, and Pavel Kulchenko (O’Reilly) is also an excellent source
of information on the SOAP protocol.
† The HTTP/1.1 specification reserves the CONNECT method but does not describe its function.
Tunnels |207
The CONNECT method asks a tunnel gateway to create a TCP connection to an
arbitrary destination server and port and to blindly relay subsequent data between
client and server.
Figure 8-10 shows how the CONNECT method works to establish a tunnel to a
gateway:
• In Figure 8-10a, the client sends a CONNECT request to the tunnel gateway.
The client’s CONNECT method asks the tunnel gateway to open a TCP connec-
tion (here, to the host named orders.joes-hardware.com on port 443, the normal
SSL port).
• The TCP connection is created in Figure 8-10b and Figure 8-10c.
• Once the TCP connection is established, the gateway notifies the client
(Figure 8-10d) by sending an HTTP 200 Connection Established response.
• At this point, the tunnel is set up. Any data sent by the client over the HTTP
tunnel will be relayed directly to the outgoing TCP connection, and any data
sent by the server will be relayed to the client over the HTTP tunnel.
Figure 8-10. Using CONNECT to establish an SSL tunnel
Client orders.joes-hardware.comGateway
(Tunnel endpoint)
The tunnel goes between client and gateway Normal SSL connection
CONNECT orders.joes-hardware.com:443 HTTP/1.0
User-agent: SuperBrowser: 4.2
mdsnrt734tngfd/p0f92piub5.
lod9fuo8w34b4/;p-90[g9yk,8
U|t6y6/%$!&9890G&*&98...
(a) CONNECT request sent
(b) Open TCP connection to port 443
(c) Connection established
HTTP/1.0 200 Connection established
(d) HTTP connection ready message returned
(e) At this point, arbitrary, bidirectional communication of raw
data occurs, until connection close
mdsnrt734tngfd/p0f92piub5.
lod9fuo8w34b4/;p-90[g9yk,8
U|t6y6/%$!&9890G&*&98... gal1304-*&hsgd
gal1304-*&hsgd
208 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
The example in Figure 8-10 describes an SSL tunnel, where SSL traffic is sent over an
HTTP connection, but the CONNECT method can be used to establish a TCP con-
nection to any server using any protocol.
CONNECT requests
The CONNECT syntax is identical in form to other HTTP methods, with the excep-
tion of the start line. The request URI is replaced by a hostname, followed by a
colon, followed by a port number. Both the host and the port must be specified:
CONNECT home.netscape.com:443 HTTP/1.0
User-agent: Mozilla/4.0
After the start line, there are zero or more HTTP request header fields, as in other
HTTP messages. As usual, the lines end in CRLFs, and the list of headers ends with a
bare CRLF.
CONNECT responses
After the request is sent, the client waits for a response from the gateway. As with
normal HTTP messages, a 200 response code indicates success. By convention, the
reason phrase in the response is normally set to “Connection Established”:
HTTP/1.0 200 Connection Established
Proxy-agent: Netscape-Proxy/1.1
Unlike normal HTTP responses, the response does not need to include a Content-
Type header. No content type is required*because the connection becomes a raw
byte relay, instead of a message carrier.
Data Tunneling, Timing, and Connection Management
Because the tunneled data is opaque to the gateway, the gateway cannot make any
assumptions about the order and flow of packets. Once the tunnel is established,
data is free to flow in any direction at any time.†
As a performance optimization, clients are allowed to send tunnel data after sending
the CONNECT request but before receiving the response. This gets data to the server
faster, but it means that the gateway must be able to handle data following the
request properly. In particular, the gateway cannot assume that a network I/O request
will return only header data, and the gateway must be sure to forward any data read
with the header to the server, when the connection is ready. Clients that pipeline data
* Future specifications may define a media type for tunnels (e.g., application/tunnel), for uniformity.
† The two endpoints of the tunnel (the client and the gateway) must be prepared to accept packets from either
of the connections at any time and must forward that data immediately. Because the tunneled protocol may
include data dependencies, neither end of the tunnel can ignore input data. Lack of data consumption on
one end of the tunnel may hang the producer on the other end of the tunnel, leading to deadlock.
Tunnels |209
after the request must be prepared to resend the request data if the response comes
back as an authentication challenge or other non-200, nonfatal status. *
If at any point either one of the tunnel endpoints gets disconnected, any outstanding
data that came from that endpoint will be passed to the other one, and after that also
the other connection will be terminated by the proxy. If there is undelivered data for
the closing endpoint, that data will be discarded.
SSL Tunneling
Web tunnels were first developed to carry encrypted SSL traffic through firewalls.
Many organizations funnel all traffic through packet-filtering routers and proxy serv-
ers to enhance security. But some protocols, such as encrypted SSL, cannot be prox-
ied by traditional proxy servers, because the information is encrypted. Tunnels let
the SSL traffic be carried through the port 80 HTTP firewall by transporting it
through an HTTP connection (Figure 8-11).
To allow SSL traffic to flow through existing proxy firewalls, a tunneling feature was
added to HTTP, in which raw, encrypted data is placed inside HTTP messages and
sent through normal HTTP channels (Figure 8-12).
* Try not to pipeline more data than can fit into the remainder of the request’s TCP packet. Pipelining more
data can cause a client TCP reset if the gateway subsequently closes the connection before all pipelined TCP
packets are received. A TCP reset can cause the client to lose the received gateway response, so the client won’t
be able to tell whether the failure was due to a network error, access control, or authentication challenge.
Figure 8-11. Tunnels let non-HTTP traffic flow through HTTP connections
Client Server
Filtering
router Firewall proxy
Filtering
router
SSL
(rejected)
(a) SSL rejected by firewall
Port 443
Client SSL server
Filtering
router Firewall proxy
Filtering
router
SSL tunneled
inside HTTP
(accepted)
(b) HTTP-carried SSL accepted by firewall
Port 80
210 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
In Figure 8-12a, SSL traffic is sent directly to a secure web server (on SSL port 443).
In Figure 8-12b, SSL traffic is encapsulated into HTTP messages and sent over HTTP
port 80 connections, until it is decapsulated back into normal SSL connections.
Tunnels often are used to let non-HTTP traffic pass through port-filtering firewalls.
This can be put to good use, for example, to allow secure SSL traffic to flow through
firewalls. However, this feature can be abused, allowing malicious protocols to flow
into an organization through the HTTP tunnel.
SSL Tunneling Versus HTTP/HTTPS Gateways
The HTTPS protocol (HTTP over SSL) can alternatively be gatewayed in the same
way as other protocols: having the gateway (instead of the client) initiate the SSL ses-
sion with the remote HTTPS server and then perform the HTTPS transaction on the
client’s part. The response will be received and decrypted by the proxy and sent to
the client over (insecure) HTTP. This is the way gateways handle FTP. However, this
approach has several disadvantages:
• The client-to-gateway connection is normal, insecure HTTP.
• The client is not able to perform SSL client authentication (authentication based
on X509 certificates) to the remote server, as the proxy is the authenticated party.
• The gateway needs to support a full SSL implementation.
Figure 8-12. Direct SSL connection vs. tunnelled SSL connection
Server
Server
Client
SSL
Tunnel start
SSLHTTP HTTP
connection SSLHTTP
SSL
Tunnel endpoint
Port 80
SSL
connection SSL
Client
SSL SSL
Port 443
Port 443
(a) Direct SSL connection
(b) SSL through HTTP tunnel
Tunnel carries SSL traffic, intended for port 443,
over a plain old HTTP connection
SSL
connection
Tunnels |211
Note that this mechanism, if used for SSL tunneling, does not require an implemen-
tation of SSL in the proxy. The SSL session is established between the client generat-
ing the request and the destination (secure) web server; the proxy server in between
merely tunnels the encrypted data and does not take any other part in the secure
transaction.
Tunnel Authentication
Other features of HTTP can be used with tunnels where appropriate. In particular,
the proxy authentication support can be used with tunnels to authenticate a client’s
right to use a tunnel (Figure 8-13).
Tunnel Security Considerations
In general, the tunnel gateway cannot verify that the protocol being spoken is really
what it is supposed to tunnel. Thus, for example, mischievous users might use tun-
nels intended for SSL to tunnel Internet gaming traffic through a corporate firewall,
Figure 8-13. Gateways can proxy-authenticate a client before it’s allowed to use a tunnel
Client orders.joes-hardware.comGateway
(Tunnel endpoint)
The tunnel goes between client and gateway Normal SLL connection
CONNECT orders.joes-hardware.com:443 HTTP/1.0
User-agent: SuperBrowser 4.2
(a) CONNECT request sent
(d) Open TCP connection to port 443
(e) Connection established
HTTP/1.0 407 Proxy authentication required
Proxy-authenticate: Basic realm="wormhole"
(b) Authentication challange returned
HTTP/1.0 200 Connection established
CONNECT orders.joes-hardware.com:443 HTTP/1.0
User-agent: SuperBrowser 4.2
Proxy-authorization: Basic YnJpYW4tdG90dHk6T3ch
(c) CONNECT request sent, with proper authorization
(f) HTTP connection ready message returned
212 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
or malicious users might use tunnels to open Telnet sessions or to send email that
bypasses corporate email scanners.
To minimize abuse of tunnels, the gateway should open tunnels only for particular
well-known ports, such as 443 for HTTPS.
Relays
HTTP relays are simple HTTP proxies that do not fully adhere to the HTTP specifi-
cations. Relays process enough HTTP to establish connections, then blindly forward
bytes.
Because HTTP is complicated, it’s sometimes useful to implement bare-bones prox-
ies that just blindly forward traffic, without performing all of the header and method
logic. Because blind relays are easy to implement, they sometimes are used to provide
simple filtering, diagnostics, or content transformation. But they should be deployed
with great caution, because of the serious potential for interoperability problems.
One of the more common (and infamous) problems with some implementations of
simple blind relays relates to their potential to cause keep-alive connections to hang,
because they don’t properly process the Connection header. This situation is depicted
in Figure 8-14.
Here’s what’s going on in this figure:
• In Figure 8-14a, a web client sends a message to the relay, including the Connec-
tion: Keep-Alive header, requesting a keep-alive connection if possible. The client
waits for a response to learn if its request for a keep-alive channel was granted.
• The relay gets the HTTP request, but it doesn’t understand the Connection
header, so it passes the message verbatim down the chain to the server
(Figure 8-14b). However, the Connection header is a hop-by-hop header; it
Figure 8-14. Simple blind relays can hang if they are single-tasking and don’t support the
Connection header
(
e
)
N
e
x
t
r
e
q
u
e
s
t
Client Server
(a) Connection: Keep-Alive
(d) Connection: Keep-Alive Blind relay
(b) Connection: Keep-Alive
(c) Connection: Keep-Alive
(f) Client’s second request on the keep-alive
connection just hangs because the relay never
processes it
(b) Server won’t close connection when done because
it thinks it has been asked to speak keep-alive
(c) Relay waits for connection to close, ignoring
any new requests on the connection
For More Information |213
applies only to a single transport link and shouldn’t be passed down the chain.
Bad things are about to start happening!
• In Figure 8-14b, the relayed HTTP request arrives at the web server. When the
web server receives the proxied Connection: Keep-Alive header, it mistakenly
concludes that the relay (which looks like any other client to the server) wants to
speak keep-alive! That’s fine with the web server—it agrees to speak keep-alive
and sends a Connection: Keep-Alive response header back in Figure 8-14c. So, at
this point, the web server thinks it is speaking keep-alive with the relay, and it
will adhere to rules of keep-alive. But the relay doesn’t know anything about
keep-alive.
• In Figure 8-14d, the relay forwards the web server’s response message back to
the client, passing along the Connection: Keep-Alive header from the web server.
The client sees this header and assumes the relay has agreed to speak keep-alive.
At this point, both the client and server believe they are speaking keep-alive, but
the relay to which they are talking doesn’t know the first thing about keep-alive.
• Because the relay doesn’t know anything about keepalive, it forwards all the data
it receives back to the client, waiting for the origin server to close the connec-
tion. But the origin server will not close the connection, because it believes the
relay asked the server to keep the connection open! So, the relay will hang wait-
ing for the connection to close.
• When the client gets the response message back in Figure 8-14d, it moves right
along to the next request, sending another request to the relay on the keep-alive
connection (Figure 8-14e). Simple relays usually never expect another request on
the same connection. The browser just spins, making no progress.
There are ways to make relays slightly smarter, to remove these risks, but any simpli-
fication of proxies runs the risk of interoperation problems. If you are building sim-
ple HTTP relays for a particular purpose, be cautious how you use them. For any
wide-scale deployment, you should strongly consider using a real, HTTP-compliant
proxy server instead.
For more information about relays and connection management, see “Keep-Alive
and Dumb Proxies” in Chapter 4.
For More Information
For more information, refer to:
http://www.w3.org/Protocols/rfc2616/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol,” by R. Fielding, J. Gettys, J. Mogul, H.
Frystyk, L. Mastinter, P. Leach, and T. Berners-Lee.
Web Proxy Servers
Ari Luotonen, Prentice Hall Computer Books.
214 |Chapter 8: Integration Points: Gateways, Tunnels, and Relays
http://www.alternic.org/drafts/drafts-l-m/draft-luotonen-web-proxy-tunneling-01.txt
“Tunneling TCP based protocols through Web proxy servers,” by Ari Luotonen.
http://cgi-spec.golux.com
The Common Gateway Interface—RFC Project Page.
http://www.w3.org/TR/2001/WD-soap12-part0-20011217/
W3C—SOAP Version 1.2 Working Draft.
Programming Web Services with SOAP
James Snell, Doug Tidwell, and Pavel Kulchenko, O’Reilly & Associates, Inc.
http://www.w3.org/TR/2002/WD-wsa-reqs-20020429
W3C—Web Services Architecture Requirements.
Web Services Essentials
Ethan Cermai, O’Reilly & Associates, Inc.
215
CHAPTER 9
Web Robots
We continue our tour of HTTP architecture with a close look at the self-animating
user agents called web robots.
Web robots are software programs that automate a series of web transactions with-
out human interaction. Many robots wander from web site to web site, fetching con-
tent, following hyperlinks, and processing the data they find. These kinds of robots
are given colorful names such as “crawlers,” “spiders,” “worms,” and “bots” because
of the way they automatically explore web sites, seemingly with minds of their own.
Here are a few examples of web robots:
• Stock-graphing robots issue HTTP GETs to stock market servers every few min-
utes and use the data to build stock price trend graphs.
• Web-census robots gather “census” information about the scale and evolution of
the World Wide Web. They wander the Web counting the number of pages and
recording the size, language, and media type of each page.*
• Search-engine robots collect all the documents they find to create search
databases.
• Comparison-shopping robots gather web pages from online store catalogs to
build databases of products and their prices.
Crawlers and Crawling
Web crawlers are robots that recursively traverse information webs, fetching first one
web page, then all the web pages to which that page points, then all the web pages to
which those pages point, and so on. When a robot recursively follows web links, it is
called a crawler or a spider because it “crawls” along the web created by HTML
hyperlinks.
*http://www.netcraft.com collects great census metrics on what flavors of servers are being used by sites
around the Web.
216 |Chapter 9: Web Robots
Internet search engines use crawlers to wander about the Web and pull back all the
documents they encounter. These documents are then processed to create a search-
able database, allowing users to find documents that contain particular words. With
billions of web pages out there to find and bring back, these search-engine spiders
necessarily are some of the most sophisticated robots. Let’s look in more detail at
how crawlers work.
Where to Start: The “Root Set”
Before you can unleash your hungry crawler, you need to give it a starting point. The
initial set of URLs that a crawler starts visiting is referred to as the root set. When
picking a root set, you should choose URLs from enough different places that crawl-
ing all the links will eventually get you to most of the web pages that interest you.
What’s a good root set to use for crawling the web in Figure 9-1? As in the real Web,
there is no single document that eventually links to every document. If you start with
document A in Figure 9-1, you can get to B, C, and D, then to E and F, then to J, and
then to K. But there’s no chain of links from A to G or from A to N.
Some web pages in this web, such as S, T, and U, are nearly stranded—isolated,
without any links pointing at them. Perhaps these lonely pages are new, and no one
has found them yet. Or perhaps they are really old or obscure.
In general, you don’t need too many pages in the root set to cover a large portion of
the web. In Figure 9-1, you need only A, G, and S in the root set to reach all pages.
Typically, a good root set consists of the big, popular web sites (for example, http://
www.yahoo.com), a list of newly created pages, and a list of obscure pages that aren’t
often linked to. Many large-scale production crawlers, such as those used by Internet
search engines, have a way for users to submit new or obscure pages into the root set.
This root set grows over time and is the seed list for any fresh crawls.
Figure 9-1. A root set is needed to reach all pages
A
B C D
E F
G
H I
L
M N
S
T U
J O
RQPK
Crawlers and Crawling |217
Extracting Links and Normalizing Relative Links
As a crawler moves through the Web, it is constantly retrieving HTML pages. It needs
to parse out the URL links in each page it retrieves and add them to the list of pages
that need to be crawled. While a crawl is progressing, this list often expands rapidly,
as the crawler discovers new links that need to be explored.*Crawlers need to do some
simple HTML parsing to extract these links and to convert relative URLs into their
absolute form. “Relative URLs” in Chapter 2 discusses how to do this conversion.
Cycle Avoidance
When a robot crawls a web, it must be very careful not to get stuck in a loop, or
cycle. Look at the crawler in Figure 9-2:
• In Figure 9-2a, the robot fetches page A, sees that A links to B, and fetches page B.
• In Figure 9-2b, the robot fetches page B, sees that B links to C, and fetches page C.
• In Figure 9-2c, the robot fetches page C and sees that C links to A. If the robot
fetches page A again, it will end up in a cycle, fetching A, B, C, A, B, C, A...
Robots must know where they’ve been to avoid cycles. Cycles can lead to robot traps
that can either halt or slow down a robot’s progress.
Loops and Dups
Cycles are bad for crawlers for at least three reasons:
• They get the crawler into a loop where it can get stuck. A loop can cause a
poorly designed crawler to spin round and round, spending all its time fetching
* In “Cycle Avoidance,” we begin to discuss the need for crawlers to remember where they have been. During
a crawl, this list of discovered URLs grows until the web space has been explored thoroughly and the crawler
reaches a point at which it is no longer discovering new links.
Figure 9-2. Crawling over a web of hyperlinks
B
C
A
BE
D
C
(a) Robot fetches page A, follows link,
fetches B
A
BE
D
C
(b) Robot follows link and fetches page C
A
BE
D
C
(c) Robot follows link and is back to A
218 |Chapter 9: Web Robots
the same pages over and over again. The crawler can burn up lots of network
bandwidth and may be completely unable to fetch any other pages.
• While the crawler is fetching the same pages repeatedly, the web server on the
other side is getting pounded. If the crawler is well connected, it can overwhelm
the web site and prevent any real users from accessing the site. Such denial of
service can be grounds for legal claims.
• Even if the looping isn’t a problem itself, the crawler is fetching a large number
of duplicate pages (often called “dups,” which rhymes with “loops”). The
crawler’s application will be flooded with duplicate content, which may make
the application useless. An example of this is an Internet search engine that
returns hundreds of matches of the exact same page.
Trails of Breadcrumbs
Unfortunately, keeping track of where you’ve been isn’t always so easy. At the time
of this writing, there are billions of distinct web pages on the Internet, not counting
content generated from dynamic gateways.
If you are going to crawl a big chunk of the world’s web content, you need to be pre-
pared to visit billions of URLs. Keeping track of which URLs have been visited can
be quite challenging. Because of the huge number of URLs, you need to use sophisti-
cated data structures to quickly determine which URLs you’ve visited. The data
structures need to be efficient in speed and memory use.
Speed is important because hundreds of millions of URLs require fast search struc-
tures. Exhaustive searching of URL lists is out of the question. At the very least, a
robot will need to use a search tree or hash table to be able to quickly determine
whether a URL has been visited.
Hundreds of millions of URLs take up a lot of space, too. If the average URL is 40
characters long, and a web robot crawls 500 million URLs (just a small portion of the
Web), a search data structure could require 20 GB or more of memory just to hold
the URLs (40 bytes per URL × 500 million URLs = 20 GB)!
Here are some useful techniques that large-scale web crawlers use to manage where
they visit:
Trees and hash tables
Sophisticated robots might use a search tree or a hash table to keep track of vis-
ited URLs. These are software data structures that make URL lookup much faster.
Lossy presence bit maps
To minimize space, some large-scale crawlers use lossy data structures such as
presence bit arrays. Each URL is converted into a fixed size number by a hash
function, and this number has an associated “presence bit” in an array. When a
Crawlers and Crawling |219
URL is crawled, the corresponding presence bit is set. If the presence bit is
already set, the crawler assumes the URL has already been crawled.*
Checkpoints
Be sure to save the list of visited URLs to disk, in case the robot program crashes.
Partitioning
As the Web grows, it may become impractical to complete a crawl with a single
robot on a single computer. That computer may not have enough memory, disk
space, computing power, or network bandwidth to complete a crawl.
Some large-scale web robots use “farms” of robots, each a separate computer,
working in tandem. Each robot is assigned a particular “slice” of URLs, for
which it is responsible. Together, the robots work to crawl the Web. The indi-
vidual robots may need to communicate to pass URLs back and forth, to cover
for malfunctioning peers, or to otherwise coordinate their efforts.
A good reference book for implementing huge data structures is Managing Gigabytes:
Compressing and Indexing Documents and Images, by Witten, et. al (Morgan Kauf-
mann). This book is full of tricks and techniques for managing large amounts of
data.
Aliases and Robot Cycles
Even with the right data structures, it is sometimes difficult to tell if you have visited
a page before, because of URL “aliasing.” Two URLs are aliases if the URLs look dif-
ferent but really refer to the same resource.
Table 9-1 illustrates a few simple ways that different URLs can point to the same
resource.
* Because there are a potentially infinite number of URLs and only a finite number of bits in the presence bit
array, there is potential for collision—two URLs can map to the same presence bit. When this happens, the
crawler mistakenly concludes that a page has been crawled when it hasn’t. In practice, this situation can be
made very unlikely by using a large number of presence bits. The penalty for collision is that a page will be
omitted from a crawl.
Table 9-1. Different URLs that alias to the same documents
First URL Second URL When aliased
ahttp://www.foo.com/bar.html http://www.foo.com:80/bar html Port is 80 by default
bhttp://www.foo.com/~fred http://www.foo.com/%7Ffred %7F is same as ~
chttp://www.foo.com/x html#early http://www.foo.com/x html#middle Tags don’t change the page
dhttp://www.foo.com/readme.htm http://www.foo.com/README.HTM Case-insensitive server
ehttp://www.foo.com/ http://www.foo.com/index.html Default page is index.html
fhttp://www.foo.com/index.html http://209.231.87.45/index.html www.foo.com has this IP address
220 |Chapter 9: Web Robots
Canonicalizing URLs
Most web robots try to eliminate the obvious aliases up front by “canonicalizing”
URLs into a standard form. A robot might first convert every URL into a canonical
form, by:
1. Adding “:80” to the hostname, if the port isn’t specified
2. Converting all %xx escaped characters into their character equivalents
3. Removing # tags
These steps can eliminate the aliasing problems shown in Table 9-1a–c. But, with-
out knowing information about the particular web server, the robot doesn’t have any
good way of avoiding the duplicates from Table 9-1d–f:
• The robot would need to know whether the web server was case-insensitive to
avoid the alias in Table 9-1d.
• The robot would need to know the web server’s index-page configuration for
this directory to know whether the URLs in Table 9-1e were aliases.
• The robot would need to know if the web server was configured to do virtual host-
ing (covered in Chapter 5) to know if the URLs in Table 9-1f were aliases, even if
it knew the hostname and IP address referred to the same physical computer.
URL canonicalization can eliminate the basic syntactic aliases, but robots will
encounter other URL aliases that can’t be eliminated through converting URLs to
standard forms.
Filesystem Link Cycles
Symbolic links on a filesystem can cause a particularly insidious kind of cycle,
because they can create an illusion of an infinitely deep directory hierarchy where
none exists. Symbolic link cycles usually are the result of an unintentional error by
the server administrator, but they also can be created by “evil webmasters” as a mali-
cious trap for robots.
Figure 9-3 shows two filesystems. In Figure 9-3a, subdir is a normal directory. In
Figure 9-3b, subdir is a symbolic link pointing back to /. In both figures, assume the
file /index.html contains a hyperlink to the file subdir/index.html.
Using Figure 9-3a’s filesystem, a web crawler may take the following actions:
1. GET http://www.foo.com/index.html
Get /index.html, find link to subdir/index.html.
2. GET http://www.foo.com/subdir/index.html
Get subdir/index.html, find link to subdir/logo.gif.
3. GET http://www.foo.com/subdir/logo.gif
Get subdir/logo.gif, no more links, all done.
Crawlers and Crawling |221
But in Figure 9-3b’s filesystem, the following might happen:
1. GET http://www.foo.com/index.html
Get /index.html, find link to subdir/index.html.
2. GET http://www.foo.com/subdir/index.html
Get subdir/index.html, but get back same index.html.
3. GET http://www.foo.com/subdir/subdir/index.html
Get subdir/subdir/index.html.
4. GET http://www.foo.com/subdir/subdir/subdir/index.html
Get subdir/subdir/subdir/index.html.
The problem with Figure 9-3b is that subdir/ is a cycle back to /, but because the
URLs look different, the robot doesn’t know from the URL alone that the docu-
ments are identical. The unsuspecting robot runs the risk of getting into a loop.
Without some kind of loop detection, this cycle will continue, often until the length
of the URL exceeds the robot’s or the server’s limits.
Dynamic Virtual Web Spaces
It’s possible for malicious webmasters to intentionally create sophisticated crawler
loops to trap innocent, unsuspecting robots. In particular, it’s easy to publish a URL
that looks like a normal file but really is a gateway application. This application can
whip up HTML on the fly that contains links to imaginary URLs on the same server.
When these imaginary URLs are requested, the nasty server fabricates a new HTML
page with new imaginary URLs.
The malicious web server can take the poor robot on an Alice-in-Wonderland jour-
ney through an infinite virtual web space, even if the web server doesn’t really con-
tain any files. Even worse, it can make it very difficult for the robot to detect the
cycle, because the URLs and HTML can look very different each time. Figure 9-4
shows an example of a malicious web server generating bogus content.
Figure 9-3. Symbolic link cycles
/
index.html subdir
index.html logo.gif
/
index.html subdir
(a) subdir is a directory (b) subdir is an upward symbolic link
222 |Chapter 9: Web Robots
More commonly, well-intentioned webmasters may unwittingly create a crawler trap
through symbolic links or dynamic content. For example, consider a CGI-based cal-
endaring program that generates a monthly calendar and a link to the next month. A
real user would not keep requesting the next-month link forever, but a robot that is
unaware of the dynamic nature of the content might keep requesting these resources
indefinitely.*
Avoiding Loops and Dups
There is no foolproof way to avoid all cycles. In practice, well-designed robots need
to include a set of heuristics to try to avoid cycles.
Generally, the more autonomous a crawler is (less human oversight), the more likely
it is to get into trouble. There is a bit of a trade-off that robot implementors need to
make—these heuristics can help avoid problems, but they also are somewhat
“lossy,” because you can end up skipping valid content that looks suspect.
Figure 9-4. Malicious dynamic web space example
* This is a real example mentioned on http://www.searchtools.com/robots/robot-checklist.html for the calendar-
ing site at http://cgi.umbc.edu/cgi-bin/WebEvent/webevent.cgi. As a result of dynamic content like this, many
robots refuse to crawl pages that have the substring “cgi” anywhere in the URL.
www.evil-joes-hardware.com
GET /index-fall.html HTTP/1.1
Host: www.evil-joes-hardware.com
Accept: *
User-agent: ShopBot
Request message
HTTP/1.1 200 OK
Content-type: text/html
Content-length: 617
<HTML><BODY>
<A HREF=/index-fall2.html>trick</A>[...]
Response message
Web robot client
www.evil-joes-hardware.com
GET /index-fall2.html HTTP/1.1
Host: www.evil-joes-hardware.com
Accept: *
User-agent: ShopBot
Request message
HTTP/1.1 200 OK
Content-type: text/html
Content-length: 617
<HTML><BODY>
<A HREF=/index-fall3.html>trick</A>[...]
Response message
Web robot client
A few sites exist that are just malicious gateway applications, whose sole purpose is to trap
unsuspecting robots with bogus content . In this example, the gateway dynamically
generates an infinite number of fake web pages, each pointing to the next.
Crawlers and Crawling |223
Some techniques that robots use to behave better in a web full of robot dangers are:
Canonicalizing URLs
Avoid syntactic aliases by converting URLs into standard form.
Breadth-first crawling
Crawlers have a large set of potential URLs to crawl at any one time. By schedul-
ing the URLs to visit in a breadth-first manner, across web sites, you can mini-
mize the impact of cycles. Even if you hit a robot trap, you still can fetch
hundreds of thousands of pages from other web sites before returning to fetch a
page from the cycle. If you operate depth-first, diving head-first into a single site,
you may hit a cycle and never escape to other sites.*
Throttling†
Limit the number of pages the robot can fetch from a web site in a period of
time. If the robot hits a cycle and continually tries to access aliases from a site,
you can cap the total number of duplicates generated and the total number of
accesses to the server by throttling.
Limit URL size
The robot may refuse to crawl URLs beyond a certain length (1KB is common).
If a cycle causes the URL to grow in size, a length limit will eventually stop the
cycle. Some web servers fail when given long URLs, and robots caught in a URL-
increasing cycle can cause some web servers to crash. This may make webmas-
ters misinterpret the robot as a denial-of-service attacker.
As a caution, this technique can certainly lead to missed content. Many sites
today use URLs to help manage user state (for example, storing user IDs in the
URLs referenced in a page). URL size can be a tricky way to limit a crawl; how-
ever, it can provide a great flag for a user to inspect what is happening on a par-
ticular site, by logging an error whenever requested URLs reach a certain size.
URL/site blacklist
Maintain a list of known sites and URLs that correspond to robot cycles and
traps, and avoid them like the plague. As new problems are found, add them to
the blacklist.
This requires human action. However, most large-scale crawlers in production
today have some form of a blacklist, used to avoid certain sites because of inher-
ent problems or something malicious in the sites. The blacklist also can be used
to avoid certain sites that have made a fuss about being crawled.‡
* Breadth-first crawling is a good idea in general, so as to more evenly disperse requests and not overwhelm
any one server. This can help keep the resources that a robot uses on a server to a minimum.
† Throttling of request rate is also discussed in “Robot Etiquette.”
‡ “Excluding Robots” discusses how sites can avoid being crawled, but some users refuse to use this simple
control mechanism and become quite irate when their sites are crawled.
224 |Chapter 9: Web Robots
Pattern detection
Cycles caused by filesystem symlinks and similar misconfigurations tend to fol-
low patterns; for example, the URL may grow with components duplicated.
Some robots view URLs with repeating components as potential cycles and
refuse to crawl URLs with more than two or three repeated components.
Not all repetition is immediate (e.g., “/subdir/subdir/subdir...”). It’s possible to
have cycles of period 2 or other intervals, such as “/subdir/images/subdir/
images/subdir/images/...”. Some robots look for repeating patterns of a few dif-
ferent periods.
Content fingerprinting
Fingerprinting is a more direct way of detecting duplicates that is used by some
of the more sophisticated web crawlers. Robots using content fingerprinting take
the bytes in the content of the page and compute a checksum. This checksum is a
compact representation of the content of the page. If a robot ever fetches a page
whose checksum it has seen before, the page’s links are not crawled—if the
robot has seen the page’s content before, it has already initiated the crawling of
the page’s links.
The checksum function must be chosen so that the odds of two different pages
having the same checksum are small. Message digest functions such as MD5 are
popular for fingerprinting.
Because some web servers dynamically modify pages on the fly, robots some-
times omit certain parts of the web page content, such as embedded links, from
the checksum calculation. Still, dynamic server-side includes that customize
arbitrary page content (adding dates, access counters, etc.) may prevent dupli-
cate detection.
Human monitoring
The Web is a wild place. Your brave robot eventually will stumble into a prob-
lem that none of your techniques will catch. All production-quality robots must
be designed with diagnostics and logging, so human beings can easily monitor
the robot’s progress and be warned quickly if something unusual is happening.
In some cases, angry net citizens will highlight the problem for you by sending
you nasty email.
Good spider heuristics for crawling datasets as vast as the Web are always works in
progress. Rules are built over time and adapted as new types of resources are added
to the Web. Good rules are always evolving.
Many smaller, more customized crawlers skirt some of these issues, as the resources
(servers, network bandwidth, etc.) that are impacted by an errant crawler are man-
ageable, or possibly even are under the control of the person performing the crawl
(such as on an intranet site). These crawlers rely on more human monitoring to pre-
vent problems.
Robotic HTTP |225
Robotic HTTP
Robots are no different from any other HTTP client program. They too need to abide
by the rules of the HTTP specification. A robot that is making HTTP requests and
advertising itself as an HTTP/1.1 client needs to use the appropriate HTTP request
headers.
Many robots try to implement the minimum amount of HTTP needed to request the
content they seek. This can lead to problems; however, it’s unlikely that this behav-
ior will change anytime soon. As a result, many robots make HTTP/1.0 requests,
because that protocol has few requirements.
Identifying Request Headers
Despite the minimum amount of HTTP that robots tend to support, most do imple-
ment and send some identification headers—most notably, the User-Agent HTTP
header. It’s recommended that robot implementors send some basic header informa-
tion to notify the site of the capabilities of the robot, the robot’s identity, and where
it originated.
This is useful information both for tracking down the owner of an errant crawler and
for giving the server some information about what types of content the robot can
handle. Some of the basic identifying headers that robot implementors are encour-
aged to implement are:
User-Agent
Tells the server the name of the robot making the request.
From
Provides the email address of the robot’s user/administrator.*
Accept
Tells the server what media types are okay to send.†This can help ensure that
the robot receives only content in which it’s interested (text, images, etc.).
Referer
Provides the URL of the document that contains the current request-URL.‡
Virtual Hosting
Robot implementors need to support the Host header. Given the prevalence of virtual
hosting (Chapter 5 discusses virtually hosted servers in more detail), not including the
* An RFC 822 email address format.
† “Accept headers” in Chapter 3 lists all of the accept headers; robots may find it useful to send headers such
as Accept-Charset if they are interested in particular versions.
‡ This can be very useful to site administrators that are trying to track down how a robot found links to their
sites’ content.
226 |Chapter 9: Web Robots
Host HTTP header in requests can lead to robots identifying the wrong content with a
particular URL. HTTP/1.1 requires the use of the Host header for this reason.
Most servers are configured to serve a particular site by default. Thus, a crawler not
including the Host header can make a request to a server serving two sites, like those
in Figure 9-5 (www.joes-hardware.com and www.foo.com) and, if the server is config-
ured to serve www.joes-hardware.com by default (and does not require the Host
header), a request for a page on www.foo.com can result in the crawler getting con-
tent from the Joe’s Hardware site. Worse yet, the crawler will actually think the con-
tent from Joe’s Hardware was from www.foo.com. I am sure you can think of some
more unfortunate situations if documents from two sites with polar political or other
views were served from the same server.
Conditional Requests
Given the enormity of some robotic endeavors, it often makes sense to minimize the
amount of content a robot retrieves. As in the case of Internet search-engine robots,
with potentially billions of web pages to download, it makes sense to re-retrieve con-
tent only if it has changed.
Some of these robots implement conditional HTTP requests,*comparing timestamps
or entity tags to see if the last version that they retrieved has been updated. This is
very similar to the way that an HTTP cache checks the validity of the local copy of a
previously fetched resource. See Chapter 7 for more on how caches validate local
copies of resources.
Figure 9-5. Example of virtual docroots causing trouble if no Host header is sent with the request
* “Conditional request headers” in Chapter 3 gives a complete listing of the conditional headers that a robot
can implement.
Robot tries to request /index.html
from www.foo.com, but does not
include a Host header.
Web robot client
www.joes-hardware.com
www.foo.com
GET /index.html HTTP/1.0
User-agent: ShopBot 1.0
Request message
HTTP/1.0 200 OK
[...]
<HTML>
<TITLE>Welcome to Joe's Hardware!</TITLE>
[...]
Response message
Server is configured to serve both sites,
but serves Joe’s Hardware by default.
Robotic HTTP |227
Response Handling
Because many robots are interested primarily in getting the content requested
through simple GET methods, often they don’t do much in the way of response han-
dling. However, robots that use some features of HTTP (such as conditional
requests), as well as those that want to better explore and interoperate with servers,
need to be able to handle different types of HTTP responses.
Status codes
In general, robots should be able to handle at least the common or expected status
codes. All robots should understand HTTP status codes such as 200 OK and 404
Not Found. They also should be able to deal with status codes that they don’t explic-
itly understand based on the general category of response. Table 3-2 in Chapter 3
gives a breakdown of the different status-code categories and their meanings.
It is important to note that some servers don’t always return the appropriate error
codes. Some servers even return 200 OK HTTP status codes with the text body of the
message describing an error! It’s hard to do much about this—it’s just something for
implementors to be aware of.
Entities
Along with information embedded in the HTTP headers, robots can look for infor-
mation in the entity itself. Meta HTML tags,*such as the meta http-equiv tag, are a
means for content authors to embed additional information about resources. The
http-equiv tag itself is a way for content authors to override certain headers that the
server handling their content may serve:
<meta http-equiv="Refresh" content="1;URL=index.html">
This tag instructs the receiver to treat the document as if its HTTP response header
contained a Refresh HTTP header with the value “1, URL=index.html”.†
Some servers actually parse the contents of HTML pages prior to sending them and
include http-equiv directives as headers; however, some do not. Robot implemen-
tors may want to scan the HEAD elements of HTML documents to look for http-
equiv information. ‡
* “Robot META directives” lists additional meta directives that site administrators and content authors can
use to control the behavior of robots and what they do with documents that have been retrieved.
† The Refresh HTTP header sometimes is used as a means to redirect users (or in this case, a robot) from one
page to another.
‡ Meta tags must occur in the HEAD section of HTML documents, according to the HTML specification.
However, they sometimes occur in other HTML document sections, as not all HTML documents adhere to
the specification.
228 |Chapter 9: Web Robots
User-Agent Targeting
Web administrators should keep in mind that many robots will visit their sites and
therefore should expect requests from them. Many sites optimize content for various
user agents, attempting to detect browser types to ensure that various site features
are supported. By doing this, the sites serve error pages instead of content to robots.
Performing a text search for the phrase “your browser does not support frames” on
some search engines will yield a list of results for error pages that contain that
phrase, when in fact the HTTP client was not a browser at all, but a robot.
Site administrators should plan a strategy for handling robot requests. For example,
instead of limiting their content development to specific browser support, they can
develop catch-all pages for non–feature rich browsers and robots. At a minimum,
they should expect robots to visit their sites and not be caught off guard when they
do.*
Misbehaving Robots
There are many ways that wayward robots can cause mayhem. Here are a few mis-
takes robots can make, and the impact of their misdeeds:
Runaway robots
Robots issue HTTP requests much faster than human web surfers, and they
commonly run on fast computers with fast network links. If a robot contains a
programming logic error, or gets caught in a cycle, it can throw intense load
against a web server—quite possibly enough to overload the server and deny ser-
vice to anyone else. All robot authors must take extreme care to design in safe-
guards to protect against runaway robots.
Stale URLs
Some robots visit lists of URLs. These lists can be old. If a web site makes a big
change in its content, robots may request large numbers of nonexistent URLs.
This annoys some web site administrators, who don’t like their error logs filling
with access requests for nonexistent documents and don’t like having their web
server capacity reduced by the overhead of serving error pages.
Long, wrong URLs
As a result of cycles and programming errors, robots may request large, non-
sense URLs from web sites. If the URL is long enough, it may reduce the perfor-
mance of the web server, clutter the web server access logs, and even cause
fragile web servers to crash.
* “Excluding Robots” provides information for how site administrators can control the behavior of robots on
their sites if there is content that should not be accessed by robots.
Excluding Robots |229
Nosy robots*
Some robots may get URLs that point to private data and make that data easily
accessible through Internet search engines and other applications. If the owner
of the data didn’t actively advertise the web pages, she may view the robotic
publishing as a nuisance at best and an invasion of privacy at worst.
Usually this happens because a hyperlink to the “private” content that the robot
followed already exists (i.e., the content isn’t as secret as the owner thought it
was, or the owner forgot to remove a preexisting hyperlink). Occasionally it hap-
pens when a robot is very zealous in trying to scavenge the documents on a site,
perhaps by fetching the contents of a directory, even if no explicit hyperlink exists.
Robot implementors retrieving large amounts of data from the Web should be
aware that their robots are likely to retrieve sensitive data at some point—data
that the site implementor never intended to be accessible over the Internet. This
sensitive data can include password files or even credit card information.
Clearly, a mechanism to disregard content once this is pointed out (and remove
it from any search index or archive) is important. Malicious search engine and
archive users have been known to exploit the abilities of large-scale web crawl-
ers to find content—some search engines, such as Google,†actually archive rep-
resentations of the pages they have crawled, so even if content is removed, it can
still be found and accessed for some time.
Dynamic gateway access
Robots don’t always know what they are accessing. A robot may fetch a URL
whose content comes from a gateway application. In this case, the data obtained
may be special-purpose and may be expensive to compute. Many web site admin-
istrators don’t like naïve robots requesting documents that come from gateways.
Excluding Robots
The robot community understood the problems that robotic web site access could
cause. In 1994, a simple, voluntary technique was proposed to keep robots out of
where they don’t belong and provide webmasters with a mechanism to better control
their behavior. The standard was named the “Robots Exclusion Standard” but is often
just called robots.txt, after the file where the access-control information is stored.
The idea of robots.txt is simple. Any web server can provide an optional file named
robots.txt in the document root of the server. This file contains information about
what robots can access what parts of the server. If a robot follows this voluntary
* Generally, if a resource is available over the public Internet, it is likely referenced somewhere. Few resources
are truly private, with the web of links that exists on the Internet.
† See search results at http://www.google.com. A cached link, which is a copy of the page that the Google
crawler retrieved and indexed, is available on most results.
230 |Chapter 9: Web Robots
standard, it will request the robots.txt file from the web site before accessing any
other resource from that site. For example, the robot in Figure 9-6 wants to down-
load http://www.joes-hardware.com/specials/acetylene-torches.html from Joe’s Hard-
ware. Before the robot can request the page, however, it needs to check the robots.txt
file to see if it has permission to fetch this page. In this example, the robots.txt file
does not block the robot, so the robot fetches the page.
The Robots Exclusion Standard
The Robots Exclusion Standard is an ad hoc standard. At the time of this writing, no
official standards body owns this standard, and vendors implement different subsets
of the standard. Still, some ability to manage robots’ access to web sites, even if
imperfect, is better than none at all, and most major vendors and search-engine
crawlers implement support for the exclusion standard.
There are three revisions of the Robots Exclusion Standard, though the naming of the
versions is not well defined. We adopt the version numbering shown in Table 9-2.
Figure 9-6. Fetching robots.txt and verifying accessibility before crawling the target file
Table 9-2. Robots Exclusion Standard versions
Version Title and description Date
0.0 A Standard for Robot Exclusion—Martijn Koster’s original robots.txt mechanism with Disallow
directive
June 1994
1.0 A Method for Web Robots Control—Martijn Koster’s IETF draft with additional support for Allow Nov. 1996
2.0 An Extended Standard for Robot Exclusion—Sean Conner’s extension including regex and timing
information; not widely supported
Nov. 1996
www.joes-hardware.comWeb robot client
GET /robots.txt
GET /specials/acetylene-torches.html
Robot parses the robots.txt file and
determines if it is allowed to access
the acetylene-torches.html file.
It is, so it proceeds with the request.
Excluding Robots |231
Most robots today adopt the v0.0 or v1.0 standards. The v2.0 standard is much more
complicated and hasn’t been widely adopted. It may never be. We’ll focus on the v1.0
standard here, because it is in wide use and is fully compatible with v0.0.
Web Sites and robots.txt Files
Before visiting any URLs on a web site, a robot must retrieve and process the robots.txt
file on the web site, if it is present.*There is a single robots.txt resource for the entire
web site defined by the hostname and port number. If the site is virtually hosted, there
can be a different robots.txt file for each virtual docroot, as with any other file.
Currently, there is no way to install “local” robots.txt files in individual subdirecto-
ries of a web site. The webmaster is responsible for creating an aggregate robots.txt
file that describes the exclusion rules for all content on the web site.
Fetching robots.txt
Robots fetch the robots.txt resource using the HTTP GET method, like any other file
on the web server. The server returns the robots.txt file, if present, in a text/plain
body. If the server responds with a 404 Not Found HTTP status code, the robot can
assume that there are no robotic access restrictions and that it can request any file.
Robots should pass along identifying information in the From and User-Agent head-
ers to help site administrators track robotic accesses and to provide contact informa-
tion in the event that the site administrator needs to inquire or complain about the
robot. Here’s an example HTTP crawler request from a commercial web robot:
GET /robots.txt HTTP/1.0
Host: www.joes-hardware.com
User-Agent: Slurp/2.0
Date: Wed Oct 3 20:22:48 EST 2001
Response codes
Many web sites do not have a robots.txt resource, but the robot doesn’t know that. It
must attempt to get the robots.txt resource from every site. The robot takes different
actions depending on the result of the robots.txt retrieval:
• If the server responds with a success status (HTTP status code 2XX), the robot
must parse the content and apply the exclusion rules to fetches from that site.
• If the server response indicates the resource does not exist (HTTP status code
404), the robot can assume that no exclusion rules are active and that access to
the site is not restricted by robots.txt.
* Even though we say “robots.txt file,” there is no reason that the robots.txt resource must strictly reside in a
filesystem. For example, the robots.txt resource could be dynamically generated by a gateway application.
232 |Chapter 9: Web Robots
• If the server response indicates access restrictions (HTTP status code 401 or 403)
the robot should regard access to the site as completely restricted.
• If the request attempt results in temporary failure (HTTP status code 503), the
robot should defer visits to the site until the resource can be retrieved.
• If the server response indicates redirection (HTTP status code 3XX), the robot
should follow the redirects until the resource is found.
robots.txt File Format
The robots.txt file has a very simple, line-oriented syntax. There are three types of
lines in a robots.txt file: blank lines, comment lines, and rule lines. Rule lines look like
HTTP headers (<Field>: <value>) and are used for pattern matching. For example:
# this robots.txt file allows Slurp & Webcrawler to crawl
# the public parts of our site, but no other robots...
User-Agent: slurp
User-Agent: webcrawler
Disallow: /private
User-Agent: *
Disallow:
The lines in a robots.txt file are logically separated into “records.” Each record
describes a set of exclusion rules for a particular set of robots. This way, different
exclusion rules can be applied to different robots.
Each record consists of a set of rule lines, terminated by a blank line or end-of-file
character. A record starts with one or more User-Agent lines, specifying which robots
are affected by this record, followed by Disallow and Allow lines that say what URLs
these robots can access.*
The previous example shows a robots.txt file that allows the Slurp and Webcrawler
robots to access any file except those files in the private subdirectory. The same file
also prevents any other robots from accessing anything on the site.
Let’s look at the User-Agent, Disallow, and Allow lines.
The User-Agent line
Each robot’s record starts with one or more User-Agent lines, of the form:
User-Agent: <robot-name>
or:
User-Agent: *
* For practical reasons, robot software should be robust and flexible with the end-of-line character. CR, LF,
and CRLF should all be supported.
Excluding Robots |233
The robot name (chosen by the robot implementor) is sent in the User-Agent header
of the robot’s HTTP GET request.
When a robot processes a robots.txt file, it must obey the record with either:
• The first robot name that is a case-insensitive substring of the robot’s name
• The first robot name that is “*”
If the robot can’t find a User-Agent line that matches its name, and can’t find a wild-
carded “User-Agent: *” line, no record matches, and access is unlimited.
Because the robot name matches case-insensitive substrings, be careful about false
matches. For example, “User-Agent: bot” matches all the robots named Bot,Robot,
Bottom-Feeder,Spambot, and Dont-Bother-Me.
The Disallow and Allow lines
The Disallow and Allow lines immediately follow the User-Agent lines of a robot
exclusion record. They describe which URL paths are explicitly forbidden or explic-
itly allowed for the specified robots.
The robot must match the desired URL against all of the Disallow and Allow rules
for the exclusion record, in order. The first match found is used. If no match is
found, the URL is allowed.*
For an Allow/Disallow line to match a URL, the rule path must be a case-sensitive
prefix of the URL path. For example, “Disallow: /tmp” matches all of these URLs:
http://www.joes-hardware.com/tmp
http://www.joes-hardware.com/tmp/
http://www.joes-hardware.com/tmp/pliers.html
http://www.joes-hardware.com/tmpspc/stuff.txt
Disallow/Allow prefix matching
Here are a few more details about Disallow/Allow prefix matching:
• Disallow and Allow rules require case-sensitive prefix matches. The asterisk has
no special meaning (unlike in User-Agent lines), but the universal wildcarding
effect can be obtained from the empty string.
• Any “escaped” characters (%XX) in the rule path or the URL path are unes-
caped back into bytes before comparison (with the exception of %2F, the for-
ward slash, which must match exactly).
• If the rule path is the empty string, it matches everything.
Table 9-3 lists several examples of matching between rule paths and URL paths.
* The robots.txt URL always is allowed and must not appear in the Allow/Disallow rules.
234 |Chapter 9: Web Robots
Prefix matching usually works pretty well, but there are a few places where it is not
expressive enough. If there are particular subdirectories for which you also want to
disallow crawling, regardless of what the prefix of the path is, robots.txt provides no
means for this. For example, you might want to avoid crawling of RCS version con-
trol subdirectories. Version 1.0 of the robots.txt scheme provides no way to support
this, other than separately enumerating every path to every RCS subdirectory.
Other robots.txt Wisdom
Here are some other rules with respect to parsing the robots.txt file:
• The robots.txt file may contain fields other than User-Agent, Disallow, and
Allow, as the specification evolves. A robot should ignore any field it doesn’t
understand.
• For backward compatibility, breaking of lines is not allowed.
• Comments are allowed anywhere in the file; they consist of optional whitespace,
followed by a comment character (#) followed by the comment, until the end-of-
line character.
• Version 0.0 of the Robots Exclusion Standard didn’t support the Allow line.
Some robots implement only the Version 0.0 specification and ignore Allow
lines. In this situation, a robot will behave conservatively, not retrieving URLs
that are permitted.
Caching and Expiration of robots.txt
If a robot had to refetch a robots.txt file before every file access, it would double the
load on web servers, as well as making the robot less efficient. Instead, robots are
expected to fetch the robots.txt file periodically and cache the results. The cached copy
of robots.txt should be used by the robot until the robots.txt file expires. Standard
HTTP cache-control mechanisms are used by both the origin server and robots to
Table 9-3. Robots.txt path matching examples
Rule path URL path Match? Comments
/tmp /tmp ✓Rule path == URL path
/tmp /tmpfile.html ✓Rule path is a prefix of URL path
/tmp /tmp/a html ✓Rule path is a prefix of URL path
/tmp/ /tmp ✗/tmp/ is not a prefix of /tmp
README.TXT ✓Empty rule path matches everything
/~fred/hi.html %7Efred/hi.html ✓%7E is treated the same as ~
/%7Efred/hi.html /~fred/hi.html ✓%7E is treated the same as ~
/%7efred/hi.html /%7Efred/hi.html ✓Case isn’t significant in escapes
/~fred/hi.html ~fred%2Fhi html ✗%2F is slash, but slash is a special case that must match exactly
Excluding Robots |235
control the caching of the robots.txt file. Robots should take note of Cache-Control
and Expires headers in the HTTP response.*
Many production crawlers today are not HTTP/1.1 clients; webmasters should note
that those crawlers will not necessarily understand the caching directives provided
for the robots.txt resource.
If no Cache-Control directives are present, the draft specification allows caching for
seven days. But, in practice, this often is too long. Web server administrators who
did not know about robots.txt often create one in response to a robotic visit, but if
the lack of a robots.txt file is cached for a week, the newly created robots.txt file will
appear to have no effect, and the site administrator will accuse the robot administra-
tor of not adhering to the Robots Exclusion Standard.†
Robot Exclusion Perl Code
A few publicly available Perl libraries exist to interact with robots.txt files. One exam-
ple is the WWW::RobotsRules module available for the CPAN public Perl archive.
The parsed robots.txt file is kept in the WWW::RobotRules object, which provides
methods to check if access to a given URL is prohibited. The same WWW::
RobotRules object can parse multiple robots.txt files.
Here are the primary methods in the WWW::RobotRules API:
Create RobotRules object
$rules = WWW::RobotRules->new($robot_name);
Load the robots.txt file
$rules->parse($url, $content, $fresh_until);
Check if a site URL is fetchable
$can_fetch = $rules->allowed($url);
Here’s a short Perl program that demonstrates the use of WWW::RobotRules:
require WWW::RobotRules;
# Create the RobotRules object, naming the robot "SuperRobot"
my $robotsrules = new WWW::RobotRules 'SuperRobot/1.0';
use LWP::Simple qw(get);
# Get and parse the robots.txt file for Joe's Hardware, accumulating the rules
$url = "http://www.joes-hardware.com/robots.txt";
my $robots_txt = get $url;
$robotsrules->parse($url, $robots_txt);
* See “Keeping Copies Fresh” in Chapter 7 for more on handling caching directives.
† Several large-scale web crawlers use the rule of refetching robots.txt daily when actively crawling the Web.
236 |Chapter 9: Web Robots
# Get and parse the robots.txt file for Mary's Antiques, accumulating the rules
$url = "http://www.marys-antiques.com/robots.txt";
my $robots_txt = get $url;
$robotsrules->parse($url, $robots_txt);
# Now RobotRules contains the set of robot exclusion rules for several
# different sites. It keeps them all separate. Now we can use RobotRules
# to test if a robot is allowed to access various URLs.
if ($robotsrules->allowed($some_target_url))
{
$c = get $url;
...
}
The following is a hypothetical robots.txt file for www.marys-antiques.com:
#####################################################################
# This is the robots.txt file for Mary's Antiques web site
#####################################################################
# Keep Suzy's robot out of all the dynamic URLs because it doesn't
# understand them, and out of all the private data, except for the
# small section Mary has reserved on the site for Suzy.
User-Agent: Suzy-Spider
Disallow: /dynamic
Allow: /private/suzy-stuff
Disallow: /private
# The Furniture-Finder robot was specially designed to understand
# Mary's antique store's furniture inventory program, so let it
# crawl that resource, but keep it out of all the other dynamic
# resources and out of all the private data.
User-Agent: Furniture-Finder
Allow: /dynamic/check-inventory
Disallow: /dynamic
Disallow: /private
# Keep everyone else out of the dynamic gateways and private data.
User-Agent: *
Disallow: /dynamic
Disallow: /private
This robots.txt file contains a record for the robot called SuzySpider, a record for the
robot called FurnitureFinder, and a default record for all other robots. Each record
applies a different set of access policies to the different robots:
• The exclusion record for SuzySpider keeps the robot from crawling the store
inventory gateway URLs that start with /dynamic and out of the private user
data, except for the section reserved for Suzy.
Excluding Robots |237
• The record for the FurnitureFinder robot permits the robot to crawl the furni-
ture inventory gateway URL. Perhaps this robot understands the format and
rules of Mary’s gateway.
• All other robots are kept out of all the dynamic and private web pages, though
they can crawl the remainder of the URLs.
Table 9-4 lists some examples for different robot accessibility to the Mary’s Antiques
web site.
HTML Robot-Control META Tags
The robots.txt file allows a site administrator to exclude robots from some or all of a
web site. One of the disadvantages of the robots.txt file is that it is owned by the web
site administrator, not the author of the individual content.
HTML page authors have a more direct way of restricting robots from individual
pages. They can add robot-control tags to the HTML documents directly. Robots
that adhere to the robot-control HTML tags will still be able to fetch the documents,
but if a robot exclusion tag is present, they will disregard the documents. For exam-
ple, an Internet search-engine robot would not include the document in its search
index. As with the robots.txt standard, participation is encouraged but not enforced.
Robot exclusion tags are implemented using HTML META tags, using the form:
<META NAME="ROBOTS" CONTENT=directive-list>
Robot META directives
There are several types of robot META directives, and new directives are likely to
be added over time and as search engines and their robots expand their activities
and feature sets. The two most-often-used robot META directives are:
NOINDEX
Tells a robot not to process the page’s content and to disregard the document
(i.e., not include the content in any index or database).
<META NAME="ROBOTS" CONTENT="NOINDEX">
Table 9-4. Robot accessibility to the Mary’s Antiques web site
URL SuzySpider FurnitureFinder NosyBot
http://www.marys-antiques.com/ ✓✓ ✓
http://www.marys-antiques.com/index.html ✓✓ ✓
http://www.marys-antiques.com/private/payroll.xls ✗✗ ✗
http://www.marys-antiques.com/private/suzy-stuff/taxes.txt ✓✗ ✗
http://www.marys-antiques.com/dynamic/buy-stuff?id=3546 ✗✗ ✗
http://www.marys-antiques.com/dynamic/check-inventory?kitchen ✗✓ ✗
238 |Chapter 9: Web Robots
NOFOLLOW
Tells a robot not to crawl any outgoing links from the page.
<META NAME="ROBOTS" CONTENT="NOFOLLOW">
In addition to NOINDEX and NOFOLLOW, there are the opposite INDEX and
FOLLOW directives, the NOARCHIVE directive, and the ALL and NONE direc-
tives. These robot META tag directives are summarized as follows:
INDEX
Tells a robot that it may index the contents of the page.
FOLLOW
Tells a robot that it may crawl any outgoing links in the page.
NOARCHIVE
Tells a robot that it should not cache a local copy of the page.*
ALL
Equivalent to INDEX, FOLLOW.
NONE
Equivalent to NOINDEX, NOFOLLOW.
The robot META tags, like all HTML META tags, must appear in the HEAD section
of an HTML page:
<html>
<head>
<meta name="robots" content="noindex,nofollow">
<title>...</title>
</head>
<body>
...
</body>
</html>
Note that the “robots” name of the tag and the content are case-insensitive.
You obviously should not specify conflicting or repeating directives, such as:
<meta name="robots" content="INDEX,NOINDEX,NOFOLLOW,FOLLOW,FOLLOW">
the behavior of which likely is undefined and certainly will vary from robot imple-
mentation to robot implementation.
Search engine META tags
We just discussed robots META tags, used to control the crawling and indexing
activity of web robots. All robots META tags contain the name="robots" attribute.
* This META tag was introduced by the folks who run the Google search engine as a way for webmasters to
opt out of allowing Google to serve cached pages of their content. It also can be used with META
NAME="googlebot".
Robot Etiquette |239
Many other types of META tags are available, including those shown in Table 9-5.
The DESCRIPTION and KEYWORDS META tags are useful for content-indexing
search-engine robots.
Robot Etiquette
In 1993, Martijn Koster, a pioneer in the web robot community, wrote up a list of
guidelines for authors of web robots. While some of the advice is dated, much of it
still is quite useful. Martijn’s original treatise, “Guidelines for Robot Writers,” can be
found at http://www.robotstxt.org/wc/guidelines.html.
Table 9-6 provides a modern update for robot designers and operators, based
heavily on the spirit and content of the original list. Most of these guidelines are tar-
geted at World Wide Web robots; however, they are applicable to smaller-scale
crawlers too.
Table 9-5. Additional META tag directives
name= content= Description
DESCRIPTION <text> Allows an author to define a short text summary of the web page. Many search engines
look at META DESCRIPTION tags, allowing page authors to specify appropriate short
abstracts to describe their web pages.
<meta name="description"
content="Welcome to Mary's Antiques web site">
KEYWORDS <comma list> Associates a comma-separated list of words that describe the web page, to assist in
keyword searches.
<meta name="keywords"
content="antiques,mary,furniture,restoration">
REVISIT-AFTER a
aThis directive is not likely to have wide support.
<no. days> Instructs the robot or search engine that the page should be revisited, presumably
because it is subject to change, after the specified number of days.
<meta name="revisit-after" content="10 days">
Table 9-6. Guidelines for web robot operators
Guideline Description
(1) Identification
Identify Your Robot Use the HTTP User-Agent field to tell web serversthe name of yourrobot. This will helpadminis-
trators understand what your robot is doing. Some robots also include a URL describing the pur-
pose and policies of the robot in the User-Agent header.
Identify Your Machine Make sure your robot runs from a machine with a DNS entry, so web sites can reverse-DNS the
robot IP address into a hostname. This will help the administrator identify the organization
responsible for the robot.
Identify a Contact Use the HTTP From field to provide a contact email address.
240 |Chapter 9: Web Robots
(2) Operations
Be Alert Your robot will generate questions and complaints. Some of this is caused by robots that run
astray. You must be cautious and watchful that your robot is behaving correctly. If your robot
runs around the clock, you need to be extra careful. You may need to have operations people
monitoring the robot 24 ×7 until your robot is well seasoned.
Be Prepared When you begin a major robotic journey, be sure to notify people at your organization. Your
organization will want to watch for network bandwidth consumption and be ready for any pub-
lic inquiries.
Monitor and Log Your robot should be richly equipped with diagnostics and logging, so you can track progress,
identify any robot traps, and sanity check that everything is working right. We cannot stress
enough the importance of monitoring and logging a robot’s behavior. Problems and complaints
will arise, and having detailed logs of a crawler’s behavior can help a robot operator backtrack to
what has happened. This is important not only for debugging your errant web crawler but also
for defending its behavior against unjustified complaints.
Learn and Adapt Each crawl, you will learn new things. Adapt your robot so it improves each time and avoids the
common pitfalls.
(3) Limit Yourself
Filter on URL If a URL looks like it refers to data that you don’t understand or are not interested in, you might
want to skip it. For example, URLs ending in “.Z”, “.gz”, “.tar”, or “.zip” are likely to be com-
pressed files or archives. URLs ending in “.exe” are likely to be programs. URLs ending in “.gif”,
“.tif”, “.jpg” are likely to be images. Make sure you get what you are after.
Filter Dynamic URLs Usually, robots don’t want to crawl content from dynamic gateways. The robot won’t know how
to properly format and post queries to gateways, and the results are likely to be erratic or tran-
sient. If a URL contains “cgi” or has a “?”, the robot may want to avoid crawling the URL.
Filter with Accept Headers Your robot should use HTTP Accept headers to tell servers what kind of content it understands.
Adhere to robots.txt Your robot should adhere to the robots.txt controls on the site.
Throttle Yourself Your robot should count the number of accesses to each site and when they occurred, and use
this information to ensure that it doesn’t visit any site too frequently. When a robot accesses a
site more frequently than every few minutes, administrators get suspicious. When a robot
accesses a site every few seconds, some administrators get angry. When a robot hammers a site
as fast as it can, shutting out all other traffic, administrators will be furious.
In general, you should limit your robot to a few requests per minute maximum, and ensure a
few seconds between each request. You also should limit the total number of accesses to a site,
to prevent loops.
(4) Tolerate Loops and Dups and Other Problems
Handle All Return Codes You must beprepared tohandle all HTTP status codes, including redirectsand errors.You should
also log and monitor these codes. A large number of non-success results on a site should cause
investigation. It may be that many URLs are stale, or the server refuses to serve documents to
robots.
Canonicalize URLs Try to remove common aliases by normalizing all URLs into a standard form.
Aggressively Avoid Cycles Work very hard to detect and avoid cycles. Treat the process of operating a crawl as a feedback
loop. The results of problems and their resolutions should be fed back into the next crawl, mak-
ing your crawler better with each iteration.
Table 9-6. Guidelines for web robot operators (continued)
Guideline Description
Robot Etiquette |241
Monitor for Traps Some types of cycles are intentional and malicious. These may be intentionally hard to detect.
Monitor for large numbers of accesses to a site with strange URLs. These may be traps.
Maintain a Blacklist When you find traps, cycles, broken sites,and sites thatwant your robot to stayaway, add them
to a blacklist, and don’t visit them again.
(5) Scalability
Understand Space Work out the math in advance for how large a problem you are solving. You may be surprised
how much memory your application will require to complete a robotic task, because of the huge
scale of the Web.
Understand Bandwidth Understand how much network bandwidth you have available and how much you will need to
complete your robotic task in the required time. Monitor the actual usage of network band-
width. You probably will find that the outgoing bandwidth (requests) is much smaller than the
incoming bandwidth (responses). By monitoring network usage, you also may find the potential
to better optimize your robot, allowing it to take better advantage of the network bandwidth by
better usage of its TCP connections.a
Understand Time Understand howlong it shouldtake for yourrobot to completeits task, andsanity check thatthe
progress matches your estimate. If your robot is way off your estimate, there probably is a prob-
lem worth investigating.
Divide and Conquer For large-scale crawls, you will likely need to apply more hardware to get the job done, either
using big multiprocessor servers with multiple network cards, or using multiple smaller comput-
ers working in unison.
(6) Reliability
Test Thoroughly Test your robot thoroughly internally before unleashing it on the world. When you are ready to
test off-site, run a few, small, maiden voyages first. Collect lots of results and analyze your per-
formance and memory use, estimating how they will scale up to the larger problem.
Checkpoint Any serious robot will need to save a snapshot of its progress, from which it can restart on fail-
ure. There will be failures: you will find software bugs, and hardware will fail. Large-scale robots
can’t start from scratch each time this happens. Design in a checkpoint/restart feature from the
beginning.
Fault Resiliency Anticipate failures, and design your robot to be able to keep making progress when they occur.
(7) Public Relations
Be Prepared Your robot probably will upset a number of people. Be prepared to respond quickly to their
enquiries. Make a web page policy statement describing your robot, and include detailed
instructions on how to create a robots.txt file.
Be Understanding Some of the people who contact you about your robot will be well informed and supportive;
others will be naïve. A few will be unusually angry. Some may well seem insane. It’s generally
unproductive to argue the importance of your robotic endeavor. Explain the Robots Exclusion
Standard, and if they are still unhappy, remove the complainant URLs immediately from your
crawl and add them to the blacklist.
Be Responsive Most unhappy webmasters are just unclear about robots. If you respond immediately and pro-
fessionally, 90% of the complaints will disappear quickly. On the other hand, if you wait several
days before responding, while your robot continues to visit a site, expect to find a very vocal,
angry opponent.
aSee Chapter 4 for more on optimizing TCP performance.
Table 9-6. Guidelines for web robot operators (continued)
Guideline Description
242 |Chapter 9: Web Robots
Search Engines
The most widespread web robots are used by Internet search engines. Internet search
engines allow users to find documents about any subject all around the world.
Many of the most popular sites on the Web today are search engines. They serve as a
starting point for many web users and provide the invaluable service of helping users
find the information in which they are interested.
Web crawlers feed Internet search engines, by retrieving the documents that exist on
the Web and allowing the search engines to create indexes of what words appear in
what documents, much like the index at the back of this book. Search engines are
the leading source of web robots—let’s take a quick look at how they work.
Think Big
When the Web was in its infancy, search engines were relatively simple databases
that helped users locate documents on the Web. Today, with the billions of pages
accessible on the Web, search engines have become essential in helping Internet
users find information. They also have become quite complex, as they have had to
evolve to handle the sheer scale of the Web.
With billions of web pages and many millions of users looking for information,
search engines have to deploy sophisticated crawlers to retrieve these billions of web
pages, as well as sophisticated query engines to handle the query load that millions
of users generate.
Think about the task of a production web crawler, having to issue billions of HTTP
queries in order to retrieve the pages needed by the search index. If each request took
half a second to complete (which is probably slow for some servers and fast for oth-
ers*), that still takes (for 1 billion documents):
0.5 seconds × (1,000,000,000) / ((60 sec/day) × (60 min/hour) × (24 hour/day))
which works out to roughly 5,700 days if the requests are made sequentially! Clearly,
large-scale crawlers need to be more clever, parallelizing requests and using banks of
machines to complete the task. However, because of its scale, trying to crawl the
entire Web still is a daunting challenge.
Modern Search Engine Architecture
Today’s search engines build complicated local databases, called “full-text indexes,”
about the web pages around the world and what they contain. These indexes act as a
sort of card catalog for all the documents on the Web.
* This depends on the resources of the server, the client robot, and the network between the two.
Search Engines |243
Search-engine crawlers gather up web pages and bring them home, adding them to
the full-text index. At the same time, search-engine users issue queries against the
full-text index through web search gateways such as HotBot (http://www.hotbot.com)
or Google (http://www.google.com). Because the web pages are changing all the time,
and because of the amount of time it can take to crawl a large chunk of the Web, the
full-text index is at best a snapshot of the Web.
The high-level architecture of a modern search engine is shown in Figure 9-7.
Full-Text Index
A full-text index is a database that takes a word and immediately tells you all the
documents that contain that word. The documents themselves do not need to be
scanned after the index is created.
Figure 9-8 shows three documents and the corresponding full-text index. The full-
text index lists the documents containing each word.
For example:
• The word “a” is in documents A and B.
• The word “best” is in documents A and C.
• The word “drill” is in documents A and B.
• The word “routine” is in documents B and C.
• The word “the” is in all three documents, A, B, and C.
Figure 9-7. A production search engine contains cooperating crawlers and query gateways
User
User
User
User
Web search
gateway Full-text index
database
Web server
Web server
Web server
Search engine
crawler/indexer
Web search users Query engine Crawling and indexing
244 |Chapter 9: Web Robots
Posting the Query
When a user issues a query to a web search-engine gateway, she fills out an HTML
form and her browser sends the form to the gateway, using an HTTP GET or POST
request. The gateway program extracts the search query and converts the web UI
query into the expression used to search the full-text index.*
Figure 9-9 shows a simple user query to the www.joes-hardware.com site. The user
types “drills” into the search box form, and the browser translates this into a GET
request with the query parameter as part of the URL.†The Joe’s Hardware web
server receives the query and hands it off to its search gateway application, which
returns the resulting list of documents to the web server, which in turn formats those
results into an HTML page for the user.
Sorting and Presenting the Results
Once a search engine has used its index to determine the results of a query, the gate-
way application takes the results and cooks up a results page for the end user.
Figure 9-8. Three documents and a full-text index
* The method for passing this query is dependent on the search solution being used.
† “Query Strings” in Chapter 2 discusses the common use of the query parameter in URLs.
a
best
buy
drill
electric
fat
fire
from
have
into
know
lose
routine
the
to
today
tools
tragedy
turned
us
we
workmaster
Word Documents
AB
AC
A
AB
A
C
B
A
A
B
C
C
BC
ABC
C
AB
A
B
B
A
AC
A
We have the best tools,
like the WorkMaster 5000
electric drill. Buy a drill
from us today!
The routine fire drill
turned into tragedy today
. . .
We know the best
routine to lose fat.
A
B
C
Search Engines |245
Since many web pages can contain any given word, search engines deploy clever
algorithms to try to rank the results. For example, in Figure 9-8, the word “best”
appears in multiple documents; search engines need to know the order in which they
should present the list of result documents in order to present users with the most
relevant results. This is called relevancy ranking—the process of scoring and order-
ing a list of search results.
To better aid this process, many of the larger search engines actually use census data
collected during the crawl of the Web. For example, counting how many links point
to a given page can help determine its popularity, and this information can be used
to weight the order in which results are presented. The algorithms, tips from crawl-
ing, and other tricks used by search engines are some of their most guarded secrets.
Spoofing
Since users often get frustrated when they do not see what they are looking for in the
first few results of a search query, the order of search results can be important in
finding a site. There is a lot of incentive for webmasters to attempt to get their sites
listed near the top of the results sections for the words that they think best describe
Figure 9-9. Example search query request
Client
User fills out HTML search form
(with a GET action HTTP method)
on site in browser and hits Submit
www.joes-hardware.com
GET /search.html?query=drills HTTP/1.1
Host: www.joes-hardware.com
Accept: *
User-agent: ShopBot
Request message
HTTP/1.1 200 OK
Content-type: text/html
Content-length: 1037
<HTML>
<HEAD><TITLE>Search Results</TITLE>
<A HREF=/BD.html>Black and Decker Drills</A>
[...]
Response message
Search gateway
Query: “drills”
Results: File “BD.html”
Welcome to Joe’s Hardware
Search for: drills
Submit
246 |Chapter 9: Web Robots
their sites, particularly if the sites are commercial and are relying on users to find
them and use their services.
This desire for better listing has led to a lot of gaming of the search system and has
created a constant tug-of-war between search-engine implementors and those seek-
ing to get their sites listed prominently. Many webmasters list tons of keywords
(some irrelevant) and deploy fake pages, or spoofs—even gateway applications that
generate fake pages that may better trick the search engines’ relevancy algorithms for
particular words.
As a result of all this, search engine and robot implementors constantly have to
tweak their relevancy algorithms to better catch these spoofs.
For More Information
For more information on web clients, refer to:
http://www.robotstxt.org/wc/robots.html
The Web Robots Pages—resources for robot developers, including the registry
of Internet Robots.
http://www.searchengineworld.com
Search Engine World—resources for search engines and robots.
http://www.searchtools.com
Search Tools for Web Sites and Intranets—resources for search tools and robots.
http://search.cpan.org/doc/ILYAZ/perl_ste/WWW/RobotRules.pm
RobotRules Perl source.
http://www.conman.org/people/spc/robots2.html
An Extended Standard for Robot Exclusion.
Managing Gigabytes: Compressing and Indexing Documents and Images
Witten, I., Moffat, A., and Bell, T., Morgan Kaufmann.
247
CHAPTER 10
HTTP-NG
As this book nears completion, HTTP is celebrating its tenth birthday. And it has
been quite an accomplished decade for this Internet protocol. Today, HTTP moves
the absolute majority of digital traffic around the world.
But as HTTP grows into its teenage years it faces a few challenges. In some ways, the
pace of HTTP adoption has gotten ahead of its design. Today, people are using
HTTP as a foundation for many diverse applications, over many different network-
ing technologies.
This chapter outlines some of the trends and challenges for the future of HTTP, and
a proposal for a next-generation architecture called HTTP-NG. While the working
group for HTTP-NG has disbanded and its rapid adoption now appears unlikely, it
nonetheless outlines some potential future directions of HTTP.
HTTP’s Growing Pains
HTTP originally was conceived as a simple technique for accessing linked multime-
dia content from distributed information servers. But, over the past decade, HTTP
and its derivatives have taken on a much broader role.
HTTP/1.1 now provides tagging and fingerprinting to track document versions,
methods to support document uploading and interactions with programmatic gate-
ways, support for multilingual content, security and authentication, caching to
reduce traffic, pipelining to reduce latency, persistent connections to reduce startup
time and improve bandwidth, and range accesses to implement partial updates.
Extensions and derivatives of HTTP have gone even further, supporting document
publishing, application serving, arbitrary messaging, video streaming, and founda-
tions for wireless multimedia access. HTTP is becoming a kind of “operating sys-
tem” for distributed media applications.
248 |Chapter 10: HTTP-NG
The design of HTTP/1.1, while well considered, is beginning to show some strains as
HTTP is used more and more as a unified substrate for complex remote operations.
There are at least four areas where HTTP shows some growing pains:
Complexity
HTTP is quite complex, and its features are interdependent. It is decidedly pain-
ful and error-prone to correctly implement HTTP software, because of the com-
plex, interwoven requirements and the intermixing of connection management,
message handling, and functional logic.
Extensibility
HTTP is difficult to extend incrementally. There are many legacy HTTP applica-
tions that create incompatibilities for protocol extensions, because they contain
no technology for autonomous functionality extensions.
Performance
HTTP has performance inefficiencies. Many of these inefficiencies will become
more serious with widespread adoption of high-latency, low-throughput wire-
less access technologies.
Transport dependence
HTTP is designed around a TCP/IP network stack. While there are no restric-
tions against alternative substacks, there has been little work in this area. HTTP
needs to provide better support for alternative substacks for it to be useful as a
broader messaging platform in embedded and wireless applications.
HTTP-NG Activity
In the summer of 1997, the World Wide Web Consortium launched a special project
to investigate and propose a major new version of HTTP that would fix the prob-
lems related to complexity, extensibility, performance, and transport dependence.
This new HTTP was called HTTP: The Next Generation (HTTP-NG).
A set of HTTP-NG proposals was presented at an IETF meeting in December 1998.
These proposals outlined one possible major evolution of HTTP. This technology
has not been widely implemented (and may never be), but HTTP-NG does represent
the most serious effort toward extending the lineage of HTTP. Let’s look at HTTP-
NG in more detail.
Modularize and Enhance
The theme of HTTP-NG can be captured in three words: “modularize and enhance.”
Instead of having connection management, message handling, server processing
logic, and protocol methods all intermixed, the HTTP-NG working group proposed
modularizing the protocol into three layers, illustrated in Figure 10-1:
Distributed Objects |249
• Layer 1, the message transport layer, focuses on delivering opaque messages
between endpoints, independent of the function of the messages. The message
transport layer supports various substacks (for example, stacks for wireless envi-
ronments) and focuses on the problems of efficient message delivery and han-
dling. The HTTP-NG project team proposed a protocol called WebMUX for this
layer.
• Layer 2, the remote invocation layer, defines request/response functionality
where clients can invoke operations on server resources. This layer is indepen-
dent of message transport and of the precise semantics of the operations. It just
provides a standard way of invoking any server operation. This layer attempts to
provide an extensible, object-oriented framework more like CORBA, DCOM,
and Java RMI than like the static, server-defined methods of HTTP/1.1. The
HTTP-NG project team proposed the Binary Wire Protocol for this layer.
• Layer 3, the web application layer, provides most of the content-management
logic. All of the HTTP/1.1 methods (GET, POST, PUT, etc.), as well as the
HTTP/1.1 header parameters, are defined here. This layer also supports other
services built on top of remote invocation, such as WebDAV.
Once the HTTP components are modularized, they can be enhanced to provide bet-
ter performance and richer functionality.
Distributed Objects
Much of the philosophy and functionality goals of HTTP-NG borrow heavily from
structured, object-oriented, distributed-objects systems such as CORBA and DCOM.
Distributed-objects systems can help with extensibility and feature functionality.
A community of researchers has been arguing for a convergence between HTTP and
more sophisticated distributed-objects systems since 1996. For more information
about the merits of a distributed-objects paradigm for the Web, check out the early
paper from Xerox PARC entitled “Migrating the Web Toward Distributed Objects”
(ftp://ftp.parc.xerox.com/pub/ilu/misc/webilu.html).
Figure 10-1. HTTP-NG separates functions into layers
Web application functions
Remote operation invocation Binary Wire Protocol
Message transport WebMUX
Underlying network transport TCP/IP
Layer 3
Layer 2
Layer 1
HTTP-NG
250 |Chapter 10: HTTP-NG
The ambitious philosophy of unifying the Web and distributed objects created
resistance to HTTP-NG’s adoption in some communities. Some past distributed-
objects systems suffered from heavyweight implementation and formal complexity.
The HTTP-NG project team attempted to address some of these concerns in the
requirements.
Layer 1: Messaging
Let’s take a closer look at the three layers of HTTP-NG, starting with the lowest layer.
The message transport layer is concerned with the efficient delivery of messages, inde-
pendent of the meaning and purpose of the messages. The message transport layer
provides an API for messaging, regardless of the actual underlying network stack.
This layer focuses on improving the performance of messaging, including:
• Pipelining and batching messages to reduce round-trip latency
• Reusing connections to reduce latency and improve delivered bandwidth
• Multiplexing multiple message streams in parallel, over the same connection, to
optimize shared connections while preventing starvation of message streams
• Efficient message segmentation to make it easier to determine message boundaries
The HTTP-NG team invested much of its energy into the development of the Web-
MUX protocol for layer 1 message transport. WebMUX is a high-performance mes-
sage protocol that fragments and interleaves messages across a multiplexed TCP
connection. We discuss WebMUX in a bit more detail later in this chapter.
Layer 2: Remote Invocation
The middle layer of the HTTP-NG architecture supports remote method invocation.
This layer provides a generic request/response framework where clients invoke opera-
tions on server resources. This layer does not concern itself with the implementation
and semantics of the particular operations (caching, security, method logic, etc.); it is
concerned only with the interface to allow clients to remotely invoke server operations.
Many remote method invocation standards already are available (CORBA, DCOM,
and Java RMI, to name a few), and this layer is not intended to support every nifty
feature of these systems. However, there is an explicit goal to extend the richness of
HTTP RMI support from that provided by HTTP/1.1. In particular, there is a goal to
provide more general remote procedure call support, in an extensible, object-oriented
manner.
The HTTP-NG team proposed the Binary Wire Protocol for this layer. This protocol
supports a high-performance, extensible technology for invoking well-described
operations on a server and carrying back the results. We discuss the Binary Wire Pro-
tocol in a bit more detail later in this chapter.
WebMUX |251
Layer 3: Web Application
The web application layer is where the semantics and application-specific logic are
performed. The HTTP-NG working group shied away from the temptation to extend
the HTTP application features, focusing instead on formal infrastructure.
The web application layer describes a system for providing application-specific ser-
vices. These services are not monolithic; different APIs may be available for different
applications. For example, the web application for HTTP/1.1 would constitute a dif-
ferent application from WebDAV, though they may share some common parts. The
HTTP-NG architecture allows multiple applications to coexist at this level, sharing
underlying facilities, and provides a mechanism for adding new applications.
The philosophy of the web application layer is to provide equivalent functionality for
HTTP/1.1 and extension interfaces, while recasting them into a framework of extensi-
ble distributed objects. You can read more about the web application layer interfaces
at http://www.w3.org/Protocols/HTTP-NG/1998/08/draft-larner-nginterfaces-00.txt.
WebMUX
The HTTP-NG working group has invested much of its energy in the development of
the WebMUX standard for message transport. WebMUX is a sophisticated, high-
performance message system, where messages can be transported in parallel across a
multiplexed TCP connection. Individual message streams, produced and consumed
at different rates, can efficiently be packetized and multiplexed over a single or small
number of TCP connections (see Figure 10-2).
Here are some of the significant goals of the WebMUX protocol:
• Simple design.
• High performance.
• Multiplexing—Multiple data streams (of arbitrary higher-level protocols) can be
interleaved dynamically and efficiently over a single connection, without stalling
data waiting for slow producers.
Figure 10-2. WebMUX can multiplex multiple messages over a single connection
Message A
Message B
Message C
Message D
Message A
Message B
Message C
Message D
252 |Chapter 10: HTTP-NG
• Credit-based flow control—Data is produced and consumed at different rates,
and senders and receivers have different amounts of memory and CPU resources
available. WebMUX uses a “credit-based” flow-control scheme, where receivers
preannounce interest in receiving data to prevent resource-scarcity deadlocks.
• Alignment preserving—Data alignment is preserved in the multiplexed stream so
that binary data can be sent and processed efficiently.
• Rich functionality—The interface is rich enough to support a sockets API.
You can read more about the WebMUX Protocol at http://www.w3.org/Protocols/
MUX/WD-mux-980722.html.
Binary Wire Protocol
The HTTP-NG team proposed the Binary Wire Protocol to enhance how the next-
generation HTTP protocol supports remote operations.
HTTP-NG defines “object types” and assigns each object type a list of methods.
Each object type is assigned a URI, so its description and methods can be advertised.
In this way, HTTP-NG is proposing a more extensible and object-oriented execution
model than that provided with HTTP/1.1, where all methods were statically defined
in the servers.
The Binary Wire Protocol carries operation-invocation requests from the client to the
server and operation-result replies from the server to the client across a stateful con-
nection. The stateful connection provides extra efficiency.
Request messages contain the operation, the target object, and optional data values.
Reply messages carry back the termination status of the operation, the serial number
of the matching request (allowing arbitrary ordering of parallel requests and
responses), and optional return values. In addition to request and reply messages,
this protocol defines several internal control messages used to improve the efficiency
and robustness of the connection.
You can read more about the Binary Wire Protocol at http://www.w3.org/Protocols/
HTTP-NG/1998/08/draft-janssen-httpng-wire-00.txt.
Current Status
At the end of 1998, the HTTP-NG team concluded that it was too early to bring the
HTTP-NG proposals to the IETF for standardization. There was concern that the
industry and community had not yet fully adjusted to HTTP/1.1 and that the signifi-
cant HTTP-NG rearchitecture to a distributed-objects paradigm would have been
extremely disruptive without a clear transition plan.
For More Information |253
Two proposals were made:
• Instead of attempting to promote the entire HTTP-NG rearchitecture in one
step, it was proposed to focus on the WebMUX transport technology. But at the
time of this writing, there hasn’t been sufficient interest to establish a WebMUX
working group.
• An effort was launched to investigate whether formal protocol types can be
made flexible enough for use on the Web, perhaps using XML. This is especially
important for a distributed-objects system that is extensible. This work is still in
progress.
At the time of this writing, no major driving HTTP-NG effort is underway. But, with
the ever-increasing use of HTTP, its growing use as a platform for diverse applica-
tions, and the growing adoption of wireless and consumer Internet technology, some
of the techniques proposed in the HTTP-NG effort may prove significant in HTTP’s
teenage years.
For More Information
For more information about HTTP-NG, please refer to the following detailed specifi-
cations and activity reports:
http://www.w3.org/Protocols/HTTP-NG/
HTTP-NG Working Group (Proposed), W3C Consortium Web Site.
http://www.w3.org/Protocols/MUX/WD-mux-980722.html
“The WebMUX Protocol,” by J. Gettys and H. Nielsen.
http://www.w3.org/Protocols/HTTP-NG/1998/08/draft-janssen-httpng-wire-00.txt
“Binary Wire Protocol for HTTP-NG,” by B. Janssen.
http://www.w3.org/Protocols/HTTP-NG/1998/08/draft-larner-nginterfaces-00.txt
“HTTP-NG Web Interfaces,” by D. Larner.
ftp://ftp.parc.xerox.com/pub/ilu/misc/webilu.html
“Migrating the Web Toward Distributed Objects,” by D. Larner.
PART III
Identification, Authorization,
and Security
The four chapters in Part III present a suite of techniques and technologies to track
identity, enforce security, and control access to content:
• Chapter 11, Client Identification and Cookies, talks about techniques to identify
users, so content can be personalized to the user audience.
• Chapter 12, Basic Authentication, highlights the basic mechanisms to verify user
identity. This chapter also examines how HTTP authentication interfaces with
databases.
• Chapter 13, Digest Authentication, explains digest authentication, a complex
proposed enhancement to HTTP that provides significantly enhanced security.
• Chapter 14, Secure HTTP, is a detailed overview of Internet cryptography, digi-
tal certificates, and the Secure Sockets Layer (SSL).
257
CHAPTER 11
Client Identification and Cookies
Web servers may talk to thousands of different clients simultaneously. These servers
often need to keep track of who they are talking to, rather than treating all requests
as coming from anonymous clients. This chapter discusses some of the technologies
that servers can use to identify who they are talking to.
The Personal Touch
HTTP began its life as an anonymous, stateless, request/response protocol. A request
came from a client, was processed by the server, and a response was sent back to the
client. Little information was available to the web server to determine what user sent
the request or to keep track of a sequence of requests from the visiting user.
Modern web sites want to provide a personal touch. They want to know more about
users on the other ends of the connections and be able to keep track of those users as
they browse. Popular online shopping sites like Amazon.com personalize their sites
for you in several ways:
Personal greetings
Welcome messages and page contents are generated specially for the user, to
make the shopping experience feel more personal.
Targeted recommendations
By learning about the interests of the customer, stores can suggest products that
they believe the customer will appreciate. Stores can also run birthday specials
near customers’ birthdays and other significant days.
Administrative information on file
Online shoppers hate having to fill in cumbersome address and credit card forms
over and over again. Some sites store these administrative details in a database.
Once they identify you, they can use the administrative information on file, mak-
ing the shopping experience much more convenient.
258 |Chapter 11: Client Identification and Cookies
Session tracking
HTTP transactions are stateless. Each request/response happens in isolation.
Many web sites want to build up incremental state as you interact with the site
(for example, filling an online shopping cart). To do this, web sites need a way to
distinguish HTTP transactions from different users.
This chapter summarizes a few of the techniques used to identify users in HTTP.
HTTP itself was not born with a rich set of identification features. The early web-site
designers (practical folks that they were) built their own technologies to identify
users. Each technique has its strengths and weaknesses. In this chapter, we’ll discuss
the following mechanisms to identify users:
• HTTP headers that carry information about user identity
• Client IP address tracking, to identify users by their IP addresses
• User login, using authentication to identify users
• Fat URLs, a technique for embedding identity in URLs
• Cookies, a powerful but efficient technique for maintaining persistent identity
HTTP Headers
Table 11-1 shows the seven HTTP request headers that most commonly carry infor-
mation about the user. We’ll discuss the first three now; the last four headers are
used for more advanced identification techniques that we’ll discuss later.
The From header contains the user’s email address. Ideally, this would be a viable
source of user identification, because each user would have a different email address.
However, few browsers send From headers, due to worries of unscrupulous servers
collecting email addresses and using them for junk mail distribution. In practice,
From headers are sent by automated robots or spiders so that if something goes
astray, a webmaster has someplace to send angry email complaints.
Table 11-1. HTTP headers carry clues about users
Header name Header type Description
From Request User’s email address
User-Agent Request User’s browser software
Referer Request Page user came from by following link
Authorization Request Username and password (discussed later)
Client-ip Extension (Request) Client’s IP address (discussed later)
X-Forwarded-For Extension (Request) Client’s IP address (discussed later)
Cookie Extension (Request) Server-generated ID label (discussed later)
Client IP Address |259
The User-Agent header tells the server information about the browser the user is
using, including the name and version of the program, and often information about
the operating system. This sometimes is useful for customizing content to interoper-
ate well with particular browsers and their attributes, but that doesn’t do much to
help identify the particular user in any meaningful way. Here are two User-Agent
headers, one sent by Netscape Navigator and the other by Microsoft Internet Explorer:
Navigator 6.2
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.4) Gecko/20011128
Netscape6/6.2.1
Internet Explorer 6.01
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)
The Referer header provides the URL of the page the user is coming from. The Ref-
erer header alone does not directly identify the user, but it does tell what page the
user previously visited. You can use this to better understand user browsing behav-
ior and user interests. For example, if you arrive at a web server coming from a base-
ball site, the server may infer you are a baseball fan.
The From, User-Agent, and Referer headers are insufficient for dependable identifi-
cation purposes. The remaining sections discuss more precise schemes to identify
particular users.
Client IP Address
Early web pioneers tried using the IP address of the client as a form of identification.
This scheme works if each user has a distinct IP address, if the IP address seldom (if
ever) changes, and if the web server can determine the client IP address for each
request. While the client IP address typically is not present in the HTTP headers,*
web servers can find the IP address of the other side of the TCP connection carrying
the HTTP request. For example, on Unix systems, the getpeername function call
returns the client IP address of the sending machine:
status = getpeername(tcp_connection_socket,...);
Unfortunately, using the client IP address to identify the user has numerous weak-
nesses that limit its effectiveness as a user-identification technology:
• Client IP addresses describe only the computer being used, not the user. If multi-
ple users share the same computer, they will be indistinguishable.
• Many Internet service providers dynamically assign IP addresses to users when
they log in. Each time they log in, they get a different address, so web servers
can’t assume that IP addresses will identify a user across login sessions.
* As we’ll see later, some proxies do add a Client-ip header, but this is not part of the HTTP standard.
260 |Chapter 11: Client Identification and Cookies
• To enhance security and manage scarce addresses, many users browse the Inter-
net through Network Address Translation (NAT) firewalls. These NAT devices
obscure the IP addresses of the real clients behind the firewall, converting the
actual client IP address into a single, shared firewall IP address (and different
port numbers).
• HTTP proxies and gateways typically open new TCP connections to the origin
server. The web server will see the IP address of the proxy server instead of that
of the client. Some proxies attempt to work around this problem by adding spe-
cial Client-ip or X-Forwarded-For HTTP extension headers to preserve the origi-
nal IP address (Figure 11-1). But not all proxies support this behavior.
Some web sites still use client IP addresses to keep track of the users between ses-
sions, but not many. There are too many places where IP address targeting doesn’t
work well.
A few sites even use client IP addresses as a security feature, serving documents only
to users from a particular IP address. While this may be adequate within the con-
fines of an intranet, it breaks down in the Internet, primarily because of the ease with
which IP addresses are spoofed (forged). The presence of intercepting proxies in the
path also breaks this scheme. Chapter 14 discusses much stronger schemes for con-
trolling access to privileged documents.
User Login
Rather than passively trying to guess the identity of a user from his IP address, a web
server can explicitly ask the user who he is by requiring him to authenticate (log in)
with a username and password.
To help make web site logins easier, HTTP includes a built-in mechanism to pass
username information to web sites, using the WWW-Authenticate and Authoriza-
tion headers. Once logged in, the browsers continually send this login information
with each request to the site, so the information is always available. We’ll discuss
this HTTP authentication in much more detail in Chapter 12, but let’s take a quick
look at it now.
Figure 11-1. Proxies can add extension headers to pass along the original client IP address
Client Server
Proxy server
209.172.34.56 Client-ip: 209.172.34.56
X-Forwarded-For: 209.172.34.56
56.41.11.4
User Login |261
If a server wants a user to register before providing access to the site, it can send back
an HTTP 401 Login Required response code to the browser. The browser will then
display a login dialog box and supply the information in the next request to the
browser, using the Authorization header.* This is depicted in Figure 11-2.
Here’s what’s happening in this figure:
• In Figure 11-2a, a browser makes a request from the www.joes-hardware.com site.
• The site doesn’t know the identity of the user, so in Figure 11-2b, the server
requests a login by returning the 401 Login Required HTTP response code and
Figure 11-2. Registering username using HTTP authentication headers
* To save users from having to log in for each request, most browsers will remember login information for a
site and pass in the login information for each request to the site.
Internet
Client Server
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
(a)
Internet
Client Server
HTTP/1.0 401 Login Required
WWW-authenticate: Basic realm="Plumbing and Fixtures"
(b)
Internet
Client Server
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
Authorization: Basic am910jRmdW4=
(c)
Internet
Client Server
HTTP/1.0 200 OK
Content-length: 4342
Content-type: text/html
...
(d)
262 |Chapter 11: Client Identification and Cookies
adds the WWW-Authenticate header. This causes the browser to pop up a login
dialog box.
• Once the user enters a username and a password (to sanity check his identity),
the browser repeats the original request. This time it adds an Authorization
header, specifying the username and password. The username and password are
scrambled, to hide them from casual or accidental network observers.*
• Now, the server is aware of the user’s identity.
• For future requests, the browser will automatically issue the stored username
and password when asked and will often even send it to the site when not asked.
This makes it possible to log in once to a site and have your identity maintained
through the session, by having the browser send the Authorization header as a
token of your identity on each request to the server.
However, logging in to web sites is tedious. As Fred browses from site to site, he’ll
need to log in for each site. To make matters worse, it is likely that poor Fred will
need to remember different usernames and passwords for different sites. His favorite
username, “fred,” will already have been chosen by someone else by the time he vis-
its many sites, and some sites will have different rules about the length and composi-
tion of usernames and passwords. Pretty soon, Fred will give up on the Internet and
go back to watching Oprah. The next section discusses a solution to this problem.
Fat URLs
Some web sites keep track of user identity by generating special versions of each URL
for each user. Typically, a real URL is extended by adding some state information to
the start or end of the URL path. As the user browses the site, the web server dynam-
ically generates hyperlinks that continue to maintain the state information in the
URLs.
URLs modified to include user state information are called fat URLs. The following
are some example fat URLs used in the Amazon.com e-commerce web site. Each
URL is suffixed by a user-unique identification number (002-1145265-8016838, in
this case) that helps track a user as she browses the store.
...
<a href="/exec/obidos/tg/browse/-/229220/ref=gr_gifts/002-1145265-8016838">All
Gifts</a><br>
<a href="/exec/obidos/wishlist/ref=gr_pl1_/002-1145265-8016838">Wish List</a><br>
...
<a href="http://s1.amazon.com/exec/varzea/tg/armed-forces/-//ref=gr_af_/002-1145265-
8016838">Salute Our Troops</a><br>
<a href="/exec/obidos/tg/browse/-/749188/ref=gr_p4_/002-1145265-8016838">Free
Shipping</a><br>
* As we will see in Chapter 14, the HTTP basic authentication username and password can easily be unscram-
bled by anyone who wants to go through a minimal effort. More secure techniques will be discussed later.
Cookies |263
<a href="/exec/obidos/tg/browse/-/468532/ref=gr_returns/002-1145265-8016838">Easy
Returns</a>
...
You can use fat URLs to tie the independent HTTP transactions with a web server
into a single “session” or “visit.” The first time a user visits the web site, a unique ID is
generated, it is added to the URL in a server-recognizable way, and the server redi-
rects the client to this fat URL. Whenever the server gets a request for a fat URL, it can
look up any incremental state associated with that user ID (shopping carts, profiles,
etc.), and it rewrites all outgoing hyperlinks to make them fat, to maintain the user ID.
Fat URLs can be used to identify users as they browse a site. But this technology does
have several serious problems. Some of these problems include:
Ugly URLs
The fat URLs displayed in the browser are confusing for new users.
Can’t share URLs
The fat URLs contain state information about a particular user and session. If
you mail that URL to someone else, you may inadvertently be sharing your accu-
mulated personal information.
Breaks caching
Generating user-specific versions of each URL means that there are no longer
commonly accessed URLs to cache.
Extra server load
The server needs to rewrite HTML pages to fatten the URLs.
Escape hatches
It is too easy for a user to accidentally “escape” from the fat URL session by
jumping to another site or by requesting a particular URL. Fat URLs work only if
the user strictly follows the premodified links. If the user escapes, he may lose
his progress (perhaps a filled shopping cart) and will have to start again.
Not persistent across sessions
All information is lost when the user logs out, unless he bookmarks the particu-
lar fat URL.
Cookies
Cookies are the best current way to identify users and allow persistent sessions. They
don’t suffer many of the problems of the previous techniques, but they often are used
in conjunction with those techniques for extra value. Cookies were first developed by
Netscape but now are supported by all major browsers.
Because cookies are important, and they define new HTTP headers, we’re going to
explore them in more detail than we did the previous techniques. The presence of
cookies also impacts caching, and most caches and browsers disallow caching of any
cookied content. The following sections present more details.
264 |Chapter 11: Client Identification and Cookies
Types of Cookies
You can classify cookies broadly into two types: session cookies and persistent cook-
ies. A session cookie is a temporary cookie that keeps track of settings and prefer-
ences as a user navigates a site. A session cookie is deleted when the user exits the
browser. Persistent cookies can live longer; they are stored on disk and survive
browser exits and computer restarts. Persistent cookies often are used to retain a
configuration profile or login name for a site that a user visits periodically.
The only difference between session cookies and persistent cookies is when they
expire. As we will see later, a cookie is a session cookie if its Discard parameter is set,
or if there is no Expires or Max-Age parameter indicating an extended expiration time.
How Cookies Work
Cookies are like “Hello, My Name Is” stickers stuck onto users by servers. When a
user visits a web site, the web site can read all the stickers attached to the user by
that server.
The first time the user visits a web site, the web server doesn’t know anything about
the user (Figure 11-3a). The web server expects that this same user will return again,
so it wants to “slap” a unique cookie onto the user so it can identify this user in the
future. The cookie contains an arbitrary list of name=value information, and it is
attached to the user using the Set-Cookie or Set-Cookie2 HTTP response (exten-
sion) headers.
Cookies can contain any information, but they often contain just a unique identifica-
tion number, generated by the server for tracking purposes. For example, in
Figure 11-3b, the server slaps onto the user a cookie that says id=“34294”. The
server can use this number to look up database information that the server accumu-
lates for its visitors (purchase history, address information, etc.).
However, cookies are not restricted to just ID numbers. Many web servers choose to
keep information directly in the cookies. For example:
Cookie: name="Brian Totty"; phone="555-1212"
The browser remembers the cookie contents sent back from the server in Set-Cookie
or Set-Cookie2 headers, storing the set of cookies in a browser cookie database (think
of it like a suitcase with stickers from various countries on it). When the user returns
to the same site in the future (Figure 11-3c), the browser will select those cookies
slapped onto the user by that server and pass them back in a Cookie request header.
Cookie Jar: Client-Side State
The basic idea of cookies is to let the browser accumulate a set of server-specific
information, and provide this information back to the server each time you visit.
Cookies |265
Because the browser is responsible for storing the cookie information, this system is
called client-side state. The official name for the cookie specification is the HTTP
State Management Mechanism.
Netscape Navigator cookies
Different browsers store cookies in different ways. Netscape Navigator stores cook-
ies in a single text file called cookies.txt. For example:
# Netscape HTTP Cookie File
# http://www.netscape.com/newsref/std/cookie_spec.html
# This is a generated file! Do not edit.
#
# domain allh path secure expires name value
www.fedex.com FALSE / FALSE 1136109676 cc /us/
.bankofamericaonline.com TRUE / FALSE 1009789256 state CA
.cnn.com TRUE / FALSE 1035069235 SelEdition www
secure.eepulse.net FALSE /eePulse FALSE 1007162968 cid %FE%FF%002
www.reformamt.org TRUE /forum FALSE 1033761379 LastVisit 1003520952
www.reformamt.org TRUE /forum FALSE 1033761379 UserName Guest
...
Figure 11-3. Slapping a cookie onto a user
Internet
Client Server
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
(a)
Internet
Client Server
HTTP/1.0 200 OK
Set-cookie: id="34294"; domain="joes-hardware.com"
Content-type: text/html
Content-length: 1903
...
(b)
Internet
Client Server
GET /index.html HTTP/1.0
Host: www.joes-hardware.com
Cookie: id="34294"
(c)
id=34294
Set-Cookie
id=34294
Cookie
266 |Chapter 11: Client Identification and Cookies
Each line of the text file represents a cookie. There are seven tab-separated fields:
domain
The domain of the cookie
allh
Whether all hosts in a domain get the cookie, or only the specific host named
path
The path prefix in the domain associated with the cookie
secure
Whether we should send this cookie only if we have an SSL connection
expiration
The cookie expiration date in seconds since Jan 1, 1970 00:00:00 GMT
name
The name of the cookie variable
value
The value of the cookie variable
Microsoft Internet Explorer cookies
Microsoft Internet Explorer stores cookies in individual text files in the cache direc-
tory. You can browse this directory to view the cookies, as shown in Figure 11-4.
The format of the Internet Explorer cookie files is proprietary, but many of the fields
are easily understood. Each cookie is stored one after the other in the file, and each
cookie consists of multiple lines.
The first line of each cookie in the file contains the cookie variable name. The next
line is the variable value. The third line contains the domain and path. The remain-
ing lines are proprietary data, presumably including dates and other flags.
Different Cookies for Different Sites
A browser can have hundreds or thousands of cookies in its internal cookie jar, but
browsers don’t send every cookie to every site. In fact, they typically send only two
or three cookies to each site. Here’s why:
• Moving all those cookie bytes would dramatically slow performance. Browsers
would actually be moving more cookie bytes than real content bytes!
• Most of these cookies would just be unrecognizable gibberish for most sites,
because they contain server-specific name/value pairs.
• Sending all cookies to all sites would create a potential privacy concern, with
sites you don’t trust getting information you intended only for another site.
Cookies |267
In general, a browser sends to a server only those cookies that the server generated.
Cookies generated by joes-hardware.com are sent to joes-hardware.com and not to
bobs-books.com or marys-movies.com.
Many web sites contract with third-party vendors to manage advertisements. These
advertisements are made to look like they are integral parts of the web site and do
push persistent cookies. When the user goes to a different web site serviced by the
same advertisement company, the persistent cookie set earlier is sent back again by
the browser (because the domains match). A marketing company could use this tech-
nique, combined with the Referer header, to potentially build an exhaustive data set
of user profiles and browsing habits. Modern browsers allow you to configure pri-
vacy settings to restrict third-party cookies.
Cookie Domain attribute
A server generating a cookie can control which sites get to see that cookie by adding
a Domain attribute to the Set-Cookie response header. For example, the following
HTTP response header tells the browser to send the cookie user=“mary17” to any
site in the domain .airtravelbargains.com:
Set-cookie: user="mary17"; domain="airtravelbargains.com"
Figure 11-4. Internet Explorer cookies are stored in individual text files in the cache directory
Can open MSIE cookies in
a text viewer program
Name = “session-id”
Value = “002-9351993-5692007”
Domain/path = “amazon.com”
Proprietary format for
other attributes
Each cookie file has
cookies for a particular
site; the cookies are stored
in text lines, one after the
other
MSIE stores cookies in the same location as other cached objects
Cookie
Cookie
268 |Chapter 11: Client Identification and Cookies
If the user visits www.airtravelbargains.com,specials.airtravelbargains.com, or any
site ending in .airtravelbargains.com, the following Cookie header will be issued:
Cookie: user="mary17"
Cookie Path attribute
The cookie specification even lets you associate cookies with portions of web sites.
This is done using the Path attribute, which indicates the URL path prefix where
each cookie is valid.
For example, one web server might be shared between two organizations, each hav-
ing separate cookies. The site www.airtravelbargains.com might devote part of its
web site to auto rentals—say, http://www.airtravelbargains.com/autos/—using a sep-
arate cookie to keep track of a user’s preferred car size. A special auto-rental cookie
might be generated like this:
Set-cookie: pref=compact; domain="airtravelbargains.com"; path=/autos/
If the user goes to http://www.airtravelbargains.com/specials.html, she will get only
this cookie:
Cookie: user="mary17"
But if she goes to http://www.airtravelbargains.com/autos/cheapo/index.html, she will
get both of these cookies:
Cookie: user="mary17"
Cookie: pref=compact
So, cookies are pieces of state, slapped onto the client by the servers, maintained by
the clients, and sent back to only those sites that are appropriate. Let’s look in more
detail at the cookie technology and standards.
Cookie Ingredients
There are two different versions of cookie specifications in use: Version 0 cookies
(sometimes called “Netscape cookies”), and Version 1 (“RFC 2965”) cookies. Ver-
sion 1 cookies are a less widely used extension of Version 0 cookies.
Neither the Version 0 or Version 1 cookie specification is documented as part of the
HTTP/1.1 specification. There are two primary adjunct documents that best describe
the use of cookies, summarized in Table 11-2.
Table 11-2. Cookie specifications
Title Description Location
Persistent Client State: HTTP Cookies Original Netscape cookie standard http://home.netscape.com/newsref/
std/cookie_spec html
RFC 2965: HTTP State Management
Mechanism
October 2000 cookie standard,
obsoletes RFC 2109
http://www.ietf.org/rfc/rfc2965.txt
Cookies |269
Version 0 (Netscape) Cookies
The initial cookie specification was defined by Netscape. These “Version 0” cookies
defined the Set-Cookie response header, the Cookie request header, and the fields
available for controlling cookies. Version 0 cookies look like this:
Set-Cookie: name=value [; expires=date] [; path=path] [; domain=domain] [; secure]
Cookie: name1=value1 [; name2=value2] ...
Version 0 Set-Cookie header
The Set-Cookie header has a mandatory cookie name and cookie value. It can be fol-
lowed by optional cookie attributes, separated by semicolons. The Set-Cookie fields
are described in Table 11-3.
Table 11-3. Version 0 (Netscape) Set-Cookie attributes
Set-Cookie attribute Description and examples
NAME=VALUE Mandatory. Both NAME and VALUE are sequences of characters, excluding the semicolon, comma,
equals sign, and whitespace, unless quoted in double quotes. The web server can create any
NAME=VALUE association, which will be sent back to the web server on subsequent visits to the site.
Set-Cookie: customer=Mary
Expires Optional. This attribute specifies a date string that defines the valid lifetime of that cookie. Once the
expiration date has been reached, the cookie will no longer be stored or given out. The date is for-
matted as:
Weekday, DD-Mon-YY HH:MM:SS GMT
The only legal time zone is GMT, and the separators between the elements of the date must be
dashes. If Expires is not specified, the cookie will expire when the user’s session ends.
Set-Cookie: foo=bar; expires=Wednesday, 09-Nov-99 23:12:40 GMT
Domain Optional. A browser sends the cookie only to server hostnames in the specified domain. This lets serv-
ers restrict cookies to only certain domains. A domain of “acme.com”would match hostnames “anvil.
acme.com” and “shipping.crate.acme.com”, but not “www.cnn.com”.
Only hosts within the specified domain can set a cookie for a domain, and domains must have at least
two or three periods in them to prevent domains of the form “.com”, “.edu”, and “va.us”. Any
domain that falls within the fixed set of special top-level domains listed here requires only two peri-
ods. Any other domain requires at least three. The special top-level domains are: .com, .edu, .net,
.org, .gov, .mil, .int, .biz, .info, .name, .museum, .coop, .aero, and .pro.
If the domain is not specified, it defaults to the hostname of the server that generated the Set-Cookie
response.
Set-Cookie: SHIPPING=FEDEX; domain="joes-hardware.com"
Path Optional. This attribute lets you assign cookies to particular documents on a server. If the Path
attribute is a prefix of a URL path, a cookie can be attached. The path “/foo” matches “/foobar” and
“/foo/bar.html”. The path “/” matches everything in the domain.
If the path is not specified, it is set to the path of the URL that generated the Set-Cookie response.
Set-Cookie: lastorder=00183; path=/orders
Secure Optional. If this attribute is included, a cookie will be sent only if HTTP is using an SSL secure connection.
Set-Cookie: private_id=519; secure
270 |Chapter 11: Client Identification and Cookies
Version 0 Cookie header
When a client sends requests, it includes all the unexpired cookies that match the
domain, path, and secure filters to the site. All the cookies are combined into a
Cookie header:
Cookie: session-id=002-1145265-8016838; session-id-time=1007884800
Version 1 (RFC 2965) Cookies
An extended version of cookies is defined in RFC 2965 (previously RFC 2109). This
Version 1 standard introduces the Set-Cookie2 and Cookie2 headers, but it also
interoperates with the Version 0 system.
The RFC 2965 cookie standard is a bit more complicated than the original Netscape
standard and is not yet completely supported. The major changes of RFC 2965 cook-
ies are:
• Associate descriptive text with each cookie to explain its purpose
• Support forced destruction of cookies on browser exit, regardless of expiration
• Max-Age aging of cookies in relative seconds, instead of absolute dates
• Ability to control cookies by the URL port number, not just domain and path
• The Cookie header carries back the domain, port, and path filters (if any)
• Version number for interoperability
•$ prefix in Cookie header to distinguish additional keywords from usernames
The Version 1 cookie syntax is as follows:
set-cookie = "Set-Cookie2:" cookies
cookies = 1#cookie
cookie = NAME "=" VALUE *(";" set-cookie-av)
NAME = attr
VALUE = value
set-cookie-av = "Comment" "=" value
| "CommentURL" "=" <"> http_URL <">
| "Discard"
| "Domain" "=" value
| "Max-Age" "=" value
| "Path" "=" value
| "Port" [ "=" <"> portlist <"> ]
| "Secure"
| "Version" "=" 1*DIGIT
portlist = 1#portnum
portnum = 1*DIGIT
cookie = "Cookie:" cookie-version 1*((";" | ",") cookie-value)
cookie-value = NAME "=" VALUE [";" path] [";" domain] [";" port]
cookie-version = "$Version" "=" value
NAME = attr
VALUE = value
Cookies |271
path = "$Path" "=" value
domain = "$Domain" "=" value
port = "$Port" [ "=" <"> value <"> ]
cookie2 = "Cookie2:" cookie-version
Version 1 Set-Cookie2 header
More attributes are available in the Version 1 cookie standard than in the Netscape
standard. Table 11-4 provides a quick summary of the attributes. Refer to RFC 2965
for more detailed explanation.
Table 11-4. Version 1 (RFC 2965) Set-Cookie2 attributes
Set-Cookie2 attribute Description and examples
NAME=VALUE Mandatory. The web server can create any NAME=VALUE association, which will be sent back to
the web server on subsequent visits to the site. The name must not begin with “$”, because that
character is reserved.
Version Mandatory. The value of this attribute is an integer, corresponding to the version of the cookie
specification. RFC 2965 is Version 1.
Set-Cookie2: Part="Rocket_Launcher_0001"; Version="1"
Comment Optional. This attribute documents how a server intends to use the cookie. The user can inspect this
policy to decide whether to permit a session with this cookie. The value must be in UTF-8 encoding.
CommentURL Optional. This attribute provides a URL pointer to detailed documentation about the purpose and
policy for a cookie. The user can inspect this policy to decide whether to permit a session with this
cookie.
Discard Optional. If this attribute is present, it instructs the client to discard the cookie when the client
program terminates.
Domain Optional. A browser sends the cookie only to server hostnames in the specified domain. This lets
servers restrict cookies to only certain domains. A domain of “acme.com” would match host-
names “anvil.acme.com” and “shipping.crate.acme.com”, but not “www.cnn.com”. The rules for
domain matching are basically the same as in Netscape cookies, but there are a few additional
rules. Refer to RFC 2965 for details.
Max-Age Optional. The value of this attribute is an integer that sets the lifetime of the cookie in seconds.
Clients should calculate the age of the cookie according to the HTTP/1.1 age-calculation rules.
When a cookie’s age becomes greater than the Max-Age, the client should discard the cookie. A
value of zero means the cookie with that name should be discarded immediately.
Path Optional. This attribute lets you assign cookies to particular documents on a server. If the Path
attribute is a prefix of a URL path, a cookie can be attached. The path “/foo” would match
“/foobar” and “/foo/bar.html”. The path “/” matches everything in the domain. If the path is not
specified, it is set to the path of the URL that generated the Set-Cookie response.
Port Optional. This attribute can stand alone as a keyword, or it can include a comma-separated list of
ports to which a cookie may be applied. If there is a port list, the cookie can be served only to serv-
ers whose ports match a port in the list. If the Port keyword is provided in isolation, the cookie can
be served only to the port number of the current responding server.
Set-Cookie2: foo="bar"; Version="1"; Port="80,81,8080"
Set-Cookie2: foo="bar"; Version="1"; Port
Secure Optional. If this attribute is included, a cookie will be sent only if HTTP is using an SSL secure
connection.
272 |Chapter 11: Client Identification and Cookies
Version 1 Cookie header
Version 1 cookies carry back additional information about each delivered cookie,
describing the filters each cookie passed. Each matching cookie must include any
Domain, Port, or Path attributes from the corresponding Set-Cookie2 headers.
For example, assume the client has received these five Set-Cookie2 responses in the
past from the www.joes-hardware.com web site:
Set-Cookie2: ID="29046"; Domain=".joes-hardware.com"
Set-Cookie2: color=blue
Set-Cookie2: support-pref="L2"; Domain="customer-care.joes-hardware.com"
Set-Cookie2: Coupon="hammer027"; Version="1"; Path="/tools"
Set-Cookie2: Coupon="handvac103"; Version="1"; Path="/tools/cordless"
If the client makes another request for path /tools/cordless/specials.html, it will pass
along a long Cookie2 header like this:
Cookie: $Version="1";
ID="29046"; $Domain=".joes-hardware.com";
color="blue";
Coupon="hammer027"; $Path="/tools";
Coupon="handvac103"; $Path="/tools/cordless"
Notice that all the matching cookies are delivered with their Set-Cookie2 filters, and
the reserved keywords begin with a dollar sign ($).
Version 1 Cookie2 header and version negotiation
The Cookie2 request header is used to negotiate interoperability between clients and
servers that understand different versions of the cookie specification. The Cookie2
header advises the server that the user agent understands new-style cookies and pro-
vides the version of the cookie standard supported (it would have made more sense
to call it Cookie-Version):
Cookie2: $Version="1"
If the server understands new-style cookies, it recognizes the Cookie2 header and
should send Set-Cookie2 (rather than Set-Cookie) response headers. If a client gets
both a Set-Cookie and a Set-Cookie2 header for the same cookie, it ignores the old
Set-Cookie header.
If a client supports both Version 0 and Version 1 cookies but gets a Version 0 Set-
Cookie header from the server, it should send cookies with the Version 0 Cookie
header. However, the client also should send Cookie2: $Version=“1” to give the
server indication that it can upgrade.
Cookies and Session Tracking
Cookies can be used to track users as they make multiple transactions to a web site.
E-commerce web sites use session cookies to keep track of users’ shopping carts as
they browse. Let’s take the example of the popular shopping site Amazon.com.
Cookies |273
When you type http://www.amazon.com into your browser, you start a chain of
transactions where the web server attaches identification information through a
series of redirects, URL rewrites, and cookie setting.
Figure 11-5 shows a transaction sequence captured from an Amazon.com visit:
• Figure 11-5a—Browser requests Amazon.com root page for the first time.
• Figure 11-5b—Server redirects the client to a URL for the e-commerce software.
• Figure 11-5c—Client makes a request to the redirected URL.
• Figure 11-5d—Server slaps two session cookies on the response and redirects the
user to another URL, so the client will request again with these cookies attached.
This new URL is a fat URL, meaning that some state is embedded into the URL.
If the client has cookies disabled, some basic identification can still be done as
long as the user follows the Amazon.com-generated fat URL links and doesn’t
leave the site.
• Figure 11-5e—Client requests the new URL, but now passes the two attached
cookies.
• Figure 11-5f—Server redirects to the home.html page and attaches two more
cookies.
• Figure 11-5g—Client fetches the home.html page and passes all four cookies.
• Figure 11-5h—Server serves back the content.
Cookies and Caching
You have to be careful when caching documents that are involved with cookie trans-
actions. You don’t want to assign one user some past user’s cookie or, worse, show
one user the contents of someone else’s personalized document.
The rules for cookies and caching are not well established. Here are some guiding
principles for dealing with caches:
Mark documents uncacheable if they are
The document owner knows best if a document is uncacheable. Explicitly mark
documents uncacheable if they are—specifically, use Cache-Control: no-
cache=“Set-Cookie” if the document is cacheable except for the Set-Cookie
header. The other, more general practice of using Cache-Control: public for doc-
uments that are cacheable promotes bandwidth savings in the Web.
Be cautious about caching Set-Cookie headers
If a response has a Set-Cookie header, you can cache the body (unless told other-
wise), but you should be extra cautious about caching the Set-Cookie header. If
you send the same Set-Cookie header to multiple users, you may be defeating
user targeting.
Some caches delete the Set-Cookie header before storing a response in the cache,
but that also can cause problems, because clients served from the cache will no
274 |Chapter 11: Client Identification and Cookies
longer get cookies slapped on them that they normally would without the cache.
This situation can be improved by forcing the cache to revalidate every request
with the origin server and merging any returned Set-Cookie headers with the cli-
ent response. The origin server can dictate such revalidations by adding this
header to the cached copy:
Cache-Control: must-revalidate, max-age=0
Figure 11-5. The Amazon.com web site uses session cookies to track users
Client www.amazon.com
GET / HTTP/1.0
Host: www.amazon.com
HTTP/1.1 302 Found
Location: http://www.amazon.com:80/exec/obidos/subst/home/redirect.html
GET /exec/obidos/subst/home/redirect.html HTTP/1.0
Host: www.amazon.com
HTTP/1.1 302 Found
Date: Sun, 02 Dec 2001 03:20:47 GMT
Set-cookie: session-id=002-1145265-8016838; path=/; domain=.amazon.com;
expires=Sunday, 09-Dec-2001 08:00:00 GMT
Set-cookie: session-id-time=1007884800; path=/; domain=.amazon.com;
expires=Sunday, 09-Dec-2001 08:00:00 GMT
GET /exec/obidos/subst/home/redirect.html/002-1145265-8016838 HTTP/1.0
Host: www.amazon.com
Cookie: session-id=002-1145265-8016838; session-id-time=1007884800
HTTP/1.1 302 Found
Date: Sun, 02 Dec 2001 03:45:40 GMT
Set-cookie: ubid-main=430-8248051-6231206; path=/; domain.amazon.com;
expires=Tuesday, 01-Jan-2036 08:00:01 GMT
Location: http://www.amazon.com/exec/obidos/subst/home/home.html/002-1145265-8016838
Set-cookie: x-main="hQ...Bf; path=/; domain=.amazon.com;
expires=Tuesday, 01-Jan-2036 08:00:01 GMT
GET /exec/obidos/subst/home/home.html/002-1145265-8016838 HTTP/1.0
Host: www.amazon.com
Cookie: session-id=002-1145265-8016838; session-id-time=1007884800;
ubid-main=430-8248051-6231206; x-main=hQ...Bf
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Cookies |275
More conservative caches may refuse to cache any response that has a Set-
Cookie header, even though the content may actually be cacheable. Some caches
allow modes when Set-Cookied images are cached, but not text.
Be cautious about requests with Cookie headers
When a request arrives with a Cookie header, it provides a hint that the result-
ing content might be personalized. Personalized content must be flagged
uncacheable, but some servers may erroneously not mark this content as
uncacheable.
Conservative caches may choose not to cache any document that comes in
response to a request with a Cookie header. And again, some caches allow
modes when Cookied images are cached, but not text. The more accepted pol-
icy is to cache images with Cookie headers, with the expiration time set to zero,
thus forcing a revalidate every time.
Cookies, Security, and Privacy
Cookies themselves are not believed to be a tremendous security risk, because they
can be disabled and because much of the tracking can be done through log analysis
or other means. In fact, by providing a standardized, scrutinized method for retain-
ing personal information in remote databases and using anonymous cookies as keys,
the frequency of communication of sensitive data from client to server can be
reduced.
Still, it is good to be cautious when dealing with privacy and user tracking, because
there is always potential for abuse. The biggest misuse comes from third-party web
sites using persistent cookies to track users. This practice, combined with IP
addresses and information from the Referer header, has enabled these marketing
companies to build fairly accurate user profiles and browsing patterns.
In spite of all the negative publicity, the conventional wisdom is that the session han-
dling and transactional convenience of cookies outweighs most risks, if you use cau-
tion about who you provide personal information to and review sites’ privacy
policies.
The Computer Incident Advisory Capability (part of the U.S. Department of Energy)
wrote an assessment of the overrepresented dangers of cookies in 1998. Here’s an
excerpt from that report:
CIAC I-034: Internet Cookies
(http://www.ciac.org/ciac/bulletins/i-034.shtml)
PROBLEM:
Cookies are short pieces of data used by web servers to help identify web users. The
popular concepts and rumors about what a cookie can do has reached almost mystical
proportions, frightening users and worrying their managers.
276 |Chapter 11: Client Identification and Cookies
VULNERABILITY ASSESSMENT:
The vulnerability of systems to damage or snooping by using web browser cookies is
essentially nonexistent. Cookies can only tell a web server if you have been there
before and can pass short bits of information (such as a user number) from the web
server back to itself the next time you visit. Most cookies last only until you quit
your browser and then are destroyed. A second type of cookie known as a persistent
cookie has an expiration date and is stored on your disk until that date. A
persistent cookie can be used to track a user's browsing habits by identifying him
whenever he returns to a site. Information about where you come from and what web
pages you visit already exists in a web server's log files and could also be used to
track users browsing habits, cookies just make it easier.
For More Information
Here are a few more useful sources for additional information about cookies:
Cookies
Simon St.Laurent, McGraw-Hill.
http://www.ietf.org/rfc/rfc2965.txt
RFC 2965, “HTTP State Management Mechanism” (obsoletes RFC 2109).
http://www.ietf.org/rfc/rfc2964.txt
RFC 2964, “Use of HTTP State Management.”
http://home.netscape.com/newsref/std/cookie_spec.html
This classic Netscape document, “Persistent Client State: HTTP Cookies,”
describes the original form of HTTP cookies that are still in common use today.
277
CHAPTER 12
Basic Authentication
Millions of people use the Web to perform private transactions and access private
data. The Web makes it very easy to access this information, but easy isn’t good
enough. We need assurances about who can look at our sensitive data and who can
perform our privileged transactions. Not all information is intended for the general
public.
We need to feel comfortable that unauthorized users can’t view our online travel
profiles or publish documents onto our web sites without our consent. We need to
make sure our most sensitive corporate-planning documents aren’t available to
unauthorized and potentially unscrupulous members of our organization. And we
need to feel at ease that our personal web communications with our children, our
spouses, and our secret loves all occur with a modicum of privacy.
Servers need a way to know who a user is. Once a server knows who the user is, it
can decide which transactions and resources the user can access. Authentication
means proving who you are; usually, you authenticate by providing a username and
a secret password. HTTP provides a native facility for HTTP authentication. While
it’s certainly possible to “roll your own” authentication facility on top of HTTP
forms and cookies, for many situations, HTTP’s native authentication fits the bill
nicely.
This chapter explains HTTP authentication and delves into the most common form
of HTTP authentication, basic authentication. The next chapter explains a more
powerful technique called digest authentication.
Authentication
Authentication means showing some proof of your identity. When you show a photo
ID, like a passport or a driver’s license, you are showing some proof that you are who
you claim to be. When you type a PIN number into an automatic teller machine, or
type a secret password into a computer’s dialog box, you also are proving that you
are who you claim to be.
278 |Chapter 12: Basic Authentication
Now, none of these schemes are foolproof. Passwords can be guessed or overheard,
and ID cards can be stolen or forged. But each piece of supporting evidence helps to
build a reasonable trust that you are who you say you are.
HTTP’s Challenge/Response Authentication Framework
HTTP provides a native challenge/response framework to make it easy to authenti-
cate users. HTTP’s authentication model is sketched in Figure 12-1.
Whenever a web application receives an HTTP request message, instead of acting on
the request, the server can respond with an “authentication challenge,” challenging
the user to demonstrate who she is by providing some secret information.
The user needs to attach the secret credentials (username and password) when she
repeats the request. If the credentials don’t match, the server can challenge the client
again or generate an error. If the credentials do match, the request completes normally.
Authentication Protocols and Headers
HTTP provides an extensible framework for different authentication protocols,
through a set of customizable control headers. The format and content of the headers
listed in Table 12-1 vary depending on the authentication protocol. The authentica-
tion protocol also is specified in the HTTP authentication headers.
Figure 12-1. Simplified challenge/response authentication
Internet
Client Server
Request Please give me the internal sales forecast.
Internet
Client Server
Challenge You requested a secret financial document.
Please tell me your username and password.
Internet
Client Server
Authorization Please give me the internal sales forecast.
Here is my username and password: “•••••”.
Internet
Client Server
Success OK. You have access rights.
Here is the document.
(Ask user for password)
Authentication |279
HTTP defines two official authentication protocols: basic authentication and digest
authentication. In the future, people are free to devise new protocols that use
HTTP’s challenge/response framework. The rest of this chapter explains basic
authentication. See Chapter 13 for details on digest authentication.
To make this concrete, let’s take a look at Figure 12-2.
When a server challenges a user, it returns a 401 Unauthorized response and
describes how and where to authenticate in the WWW-Authenticate header
(Figure 12-2b).
Table 12-1. Four phases of authentication
Phase Headers Description Method/Status
Request The first request has no authentication. GET
Challenge WWW-Authenticate The server rejects the request with a 401 status, indicating
that the user needs to provide his username and password.
Because the server might have different areas, each with its
own password, the server describes the protection area in
the WWW-Authenticate header. Also, the authentication
algorithm is specified in the WWW-Authenticate header.
401 Unauthorized
Authorization Authorization The client retries the request, but this time attaching an
Authorization header specifying the authentication algo-
rithm, username, and password.
GET
Success Authentication-Info If the authorization credentials are correct, the server
returns the document. Some authorization algorithms
return some additional information about the authorization
session in the optional Authentication-Info header.
200 OK
Figure 12-2. Basic authentication example
Client Server
GET /family/jeff.jpg HTTP/1.0
HTTP/1.0 401 Authorization Required
WWW-Authenticate: Basic realm="Family"
(a)
(b)
GET /family/jeff.jpg HTTP/1.0
Authorization: Basic YnJpYW4tdG90dHk6T3ch (c)
HTTP/1.0 200 OK
Content-type: image/jpeg
...<image data included>
280 |Chapter 12: Basic Authentication
When a client authorizes the server to proceed, it resends the request but attaches an
encoded password and other authentication parameters in an Authorization header
(Figure 12-2c).
When an authorized request is completed successfully, the server returns a normal
status code (e.g., 200 OK) and, for advanced authentication algorithms, might attach
additional information in an Authentication-Info header (Figure 12-2d).
Security Realms
Before we discuss the details of basic authentication, we need to explain how HTTP
allows servers to associate different access rights to different resources. You might
have noticed that the WWW-Authenticate challenge in Figure 12-2b included a
realm directive. Web servers group protected documents into security realms. Each
security realm can have different sets of authorized users.
For example, suppose a web server has two security realms established: one for cor-
porate financial information and another for personal family documents (see
Figure 12-3). Different users will have different access to the realms. The CEO of
your company probably should have access to the sales forecast, but you might not
give her access to your family vacation photos!
Here’s a hypothetical basic authentication challenge, with a realm specified:
HTTP/1.0 401 Unauthorized
WWW-Authenticate: Basic realm="Corporate Financials"
A realm should have a descriptive string name, like “Corporate Financials,” to help
the user understand which username and password to use. It may also be useful to list
the server hostname in the realm name—for example, “executive-committee@big-
company.com”.
Figure 12-3. Security realms in a web server
Family realm
/
corporate index.html
press financials
family
pr1.html pr2.html sales-forecast.xls
jeff.jpg brian.jpg
Corporate financials realm
Server
Basic Authentication |281
Basic Authentication
Basic authentication is the most prevalent HTTP authentication protocol. Almost
every major client and server implements basic authentication. Basic authentication
was originally described in the HTTP/1.0 specification, but it has since been relo-
cated into RFC 2617, which details HTTP authentication.
In basic authentication, a web server can refuse a transaction, challenging the client
for a valid username and password. The server initiates the authentication challenge
by returning a 401 status code instead of 200 and specifies the security realm being
accessed with the WWW-Authenticate response header. When the browser receives
the challenge, it opens a dialog box requesting the username and password for this
realm. The username and password are sent back to the server in a slightly scram-
bled format inside an Authorization request header.
Basic Authentication Example
Figure 12-2, earlier in this chapter, showed a detailed example of basic authentication:
• In Figure 12-2a, a user requests the personal family photo /family/jeff.jpg.
• In Figure 12-2b, the server sends back a 401 Authorization Required password
challenge for the personal family photo, along with the WWW-Authenticate
header. The header requests basic authentication for the realm named Family.
• In Figure 12-2c, the browser receives the 401 challenge and pops open a dialog
box asking for the username and password for the Family realm. When the user
enters the username and password, the browser joins them with a colon,
encodes them into a “scrambled” base-64 representation (discussed in the next
section), and sends them back in the Authorization header.
• In Figure 12-2d, the server decodes the username and password, verifies that they
are correct, and returns the requested document in an HTTP 200 OK message.
The HTTP basic authentication WWW-Authenticate and Authorization headers are
summarized in Table 12-2.
Table 12-2. Basic authentication headers
Challenge/Response Header syntax and description
Challenge (server to client) There may be different passwords for different parts of the site. The realm is a quoted string
that names the set of documents being requested, so the user knows which password to use.
WWW-Authenticate: Basic realm=quoted-realm
Response (client to server) The username and password are joined together by a colon (:) and then converted to base-64
encoding, making it a bit easier to include international characters in usernames and passwords
and making it less likely that a cursory examination will yield usernames and passwords while
watching network traffic.
Authorization: Basic base64-username-and-password
282 |Chapter 12: Basic Authentication
Note that the basic authentication protocol does not make use of the Authentication-
Info header we showed in Table 12-1.
Base-64 Username/Password Encoding
HTTP basic authentication packs the username and password together (separated by
a colon), and encodes them using the base-64 encoding method. If you don’t know
what base-64 encoding is, don’t worry. You don’t need to know much about it, and
if you are curious, you can read all about it in Appendix E. In a nutshell, base-64
encoding takes a sequence of 8-bit bytes and breaks the sequence of bits into 6-bit
chunks. Each 6-bit piece is used to pick a character in a special 64-character alpha-
bet, consisting mostly of letters and numbers.
Figure 12-4 shows an example of using base-64 encoding for basic authentication.
Here, the username is “brian-totty” and the password is “Ow!”. The browser joins the
username and password with a colon, yielding the packed string “brian-totty:Ow!”.
This string is then base 64–encoded into this mouthful: “YnJpYW4tdG90dHk6T3ch”.
Base-64 encoding was invented to take strings of binary, text, and international char-
acter data (which caused problems on some systems) and convert them temporarily
into a portable alphabet for transmission. The original strings could then be decoded
on the remote end without fear of transmission corruption.
Base-64 encoding can be useful for usernames and passwords that contain interna-
tional characters or other characters that are illegal in HTTP headers (such as quo-
tation marks, colons, and carriage returns). Also, because base-64 encoding
trivially scrambles the username and password, it can help prevent administrators
Figure 12-4. Generating a basic Authorization header from username and password
(a) Prompt for username and password
brian-totty
Ow!
(b ) Pack username and password with colon brian-totty
Ow!
(c) Base 64 encode BASE64ENC(brian-totty:Ow!)
(d) Send authorization
brian-totty:Ow!
YnJpYW4tdG90dHk6T3ch
Client Server
GET /family/jeff.jpg HTTP/1.0
Authorization: Basic YnJpYW4tdG90dHk6T3ch
The Security Flaws of Basic Authentication |283
from accidentally viewing usernames and passwords while administering servers
and networks.
Proxy Authentication
Authentication also can be done by intermediary proxy servers. Some organizations
use proxy servers to authenticate users before letting them access servers, LANs, or
wireless networks. Proxy servers can be a convenient way to provide unified access
control across an organization’s resources, because access policies can be centrally
administered on the proxy server. The first step in this process is to establish the
identity via proxy authentication.
The steps involved in proxy authentication are identical to that of web server identifi-
cation. However, the headers and status codes are different. Table 12-3 contrasts the
status codes and headers used in web server and proxy authentication.
The Security Flaws of Basic Authentication
Basic authentication is simple and convenient, but it is not secure. It should only be
used to prevent unintentional access from nonmalicious parties or used in combina-
tion with an encryption technology such as SSL.
Consider the following security flaws:
1. Basic authentication sends the username and password across the network in a
form that can trivially be decoded. In effect, the secret password is sent in the
clear, for anyone to read and capture. Base-64 encoding obscures the username
and password, making it less likely that friendly parties will glean passwords by
accidental network observation. However, given a base 64–encoded username
and password, the decoding can be performed trivially by reversing the encod-
ing process. Decoding can even be done in seconds, by hand, with pencil and
paper! Base 64–encoded passwords are effectively sent “in the clear.” Assume
that motivated third parties will intercept usernames and passwords sent by
basic authentication. If this is a concern, send all your HTTP transactions over
SSL encrypted channels, or use a more secure authentication protocol, such as
digest authentication.
Table 12-3. Web server versus proxy authentication
Web server Proxy server
Unauthorized status code: 401 Unauthorized status code: 407
WWW-Authenticate Proxy-Authenticate
Authorization Proxy-Authorization
Authentication-Info Proxy-Authentication-Info
284 |Chapter 12: Basic Authentication
2. Even if the secret password were encoded in a scheme that was more compli-
cated to decode, a third party could still capture the garbled username and pass-
word and replay the garbled information to origin servers over and over again to
gain access. No effort is made to prevent these replay attacks.
3. Even if basic authentication is used for noncritical applications, such as corpo-
rate intranet access control or personalized content, social behavior makes this
dangerous. Many users, overwhelmed by a multitude of password-protected ser-
vices, share usernames and passwords. A clever, malicious party may capture a
username and password in the clear from a free Internet email site, for example,
and find that the same username and password allow access to critical online
banking sites!
4. Basic authentication offers no protection against proxies or intermediaries that
act as middlemen, leaving authentication headers intact but modifying the rest of
the message to dramatically change the nature of the transaction.
5. Basic authentication is vulnerable to spoofing by counterfeit servers. If a user can
be led to believe that he is connecting to a valid host protected by basic authenti-
cation when, in fact, he is connecting to a hostile server or gateway, the attacker
can request a password, store it for later use, and feign an error.
This all said, basic authentication still is useful for providing convenient personaliza-
tion or access control to documents in a friendly environment, or where privacy is
desired but not absolutely necessary. In this way, basic authentication is used to pre-
vent accidental or casual access by curious users.*
For example, inside a corporation, product management may password-protect
future product plans to limit premature distribution. Basic authentication makes it
sufficiently inconvenient for friendly parties to access this data.†Likewise, you might
password-protect personal photos or private web sites that aren’t top-secret or don’t
contain valuable information, but really aren’t anyone else’s business either.
Basic authentication can be made secure by combining it with encrypted data trans-
mission (such as SSL) to conceal the username and password from malicious individ-
uals. This is a common technique.
We discuss secure encryption in Chapter 14. The next chapter explains a more
sophisticated HTTP authentication protocol, digest authentication, that has stron-
ger security properties than basic authentication.
* Be careful that the username/password in basic authentication is not the same as the password on your more
secure systems, or malicious users can use them to break into your secure accounts!
† While not very secure, internal employees of the company usually are unmotivated to maliciously capture
passwords. That said, corporate espionage does occur, and vengeful, disgruntled employees do exist, so it is
wise to place any data that would be very harmful if maliciously acquired under a stronger security scheme.
For More Information |285
For More Information
For more information on basic authentication and LDAP, see:
http://www.ietf.org/rfc/rfc2617.txt
RFC 2617, “HTTP Authentication: Basic and Digest Access Authentication.”
http://www.ietf.org/rfc/rfc2616.txt
RFC 2616 “Hypertext Transfer Protocol—HTTP/1.1.”
286
CHAPTER 13
Digest Authentication
Basic authentication is convenient and flexible but completely insecure. Usernames
and passwords are sent in the clear,*and there is no attempt to protect messages
from tampering. The only way to use basic authentication securely is to use it in con-
junction with SSL.
Digest authentication was developed as a compatible, more secure alternative to
basic authentication. We devote this chapter to the theory and practice of digest
authentication. Even though digest authentication is not yet in wide use, the con-
cepts still are important for anyone implementing secure transactions.
The Improvements of Digest Authentication
Digest authentication is an alternate HTTP authentication protocol that tries to fix
the most serious flaws of basic authentication. In particular, digest authentication:
• Never sends secret passwords across the network in the clear
• Prevents unscrupulous individuals from capturing and replaying authentication
handshakes
• Optionally can guard against tampering with message contents
• Guards against several other common forms of attacks
Digest authentication is not the most secure protocol possible.†Many needs for
secure HTTP transactions cannot be met by digest authentication. For those needs,
Transport Layer Security (TLS) and Secure HTTP (HTTPS) are more appropriate
protocols.
* Usernames and passwords are scrambled using a trivial base-64 encoding, which can be decoded easily. This
protects against unintentional accidental viewing but offers no protection against malicious parties.
† For example, compared to public key–based mechanisms, digest authentication does not provide a strong
authentication mechanism. Also, digest authentication offers no confidentiality protection beyond protect-
ing the actual password—the rest of the request and response are available to eavesdroppers.
The Improvements of Digest Authentication |287
However, digest authentication is significantly stronger than basic authentication,
which it was designed to replace. Digest authentication also is stronger than many
popular schemes proposed for other Internet services, such as CRAM-MD5, which
has been proposed for use with LDAP, POP, and IMAP.
To date, digest authentication has not been widely deployed. However, because of the
security risks inherent to basic authentication, the HTTP architects counsel in RFC
2617 that “any service in present use that uses Basic should be switched to Digest as
soon as practical.”* It is not yet clear how successful this standard will become.
Using Digests to Keep Passwords Secret
The motto of digest authentication is “never send the password across the network.”
Instead of sending the password, the client sends a “fingerprint” or “digest” of the
password, which is an irreversible scrambling of the password. The client and the
server both know the secret password, so the server can verify that the digest pro-
vided a correct match for the password. Given only the digest, a bad guy has no easy
way to find what password it came from, other than going through every password
in the universe, trying each one!†
Let’s see how this works (this is a simplified version):
• In Figure 13-1a, the client requests a protected document.
• In Figure 13-1b, the server refuses to serve the document until the client authen-
ticates its identity by proving it knows the password. The server issues a chal-
lenge to the client, asking for the username and a digested form of the password.
• In Figure 13-1c, the client proves that it knows the password by passing along
the digest of the password. The server knows the passwords for all the users,‡so
it can verify that the user knows the password by comparing the client-supplied
digest with the server’s own internally computed digest. Another party would
not easily be able to make up the right digest if it didn’t know the password.
• In Figure 13-1d, the server compares the client-provided digest with the server’s
internally computed digest. If they match, it shows that the client knows the
password (or made a really lucky guess!). The digest function can be set to gen-
erate so many digits that lucky guesses effectively are impossible. When the
server verifies the match, the document is served to the client—all without ever
sending the password over the network.
* There has been significant debate about the relevance of digest authentication, given the popularity and
widespread adoption of SSL-encrypted HTTP. Time will tell if digest authentication gains the critical mass
required.
† There are techniques, such as dictionary attacks, where common passwords are tried first. These cryptanal-
ysis techniques can dramatically ease the process of cracking passwords.
‡ In fact, the server really needs to know only the digests of the passwords.
288 |Chapter 13: Digest Authentication
We’ll discuss the particular headers used in digest authentication in more detail in
Table 13-8.
One-Way Digests
A digest is a “condensation of a body of information.”*Digests act as one-way func-
tions, typically converting an infinite number of possible input values into a finite
range of condensations.†One popular digest function, MD5,‡converts any arbitrary
sequence of bytes, of any length, into a 128-bit digest.
128 bits = 2128, or about 1,000,000,000,000,000,000,000,000,000,000,000,000,000
possible distinct condensations.
Figure 13-1. Using digests for password-obscured authentication
* Merriam-Webster dictionary, 1998.
† In theory, because we are converting an infinite number of input values into a finite number of output values,
it is possible to have two distinct inputs map to the same digest. This is called a collision. In practice, the
number of potential outputs is so large that the chance of a collision in real life is vanishingly small and, for
the purpose of password matching, unimportant.
‡ MD5 stands for “Message Digest #5,” one in a series of digest algorithms. The Secure Hash Algorithm (SHA)
is another popular digest function.
Internet
Client Server
(a) Request Please give me the internal sales forecast.
Internet
Client Server
(b) Challenge You requested a secret financial document.
Please tell me your username and
password digest.
Internet
Client Server
(c) Authorization Please give me the internal sales forecast.
My username is “bri”
My digested password is “A3F5”
Internet
Client Server
(d) Success OK. The digest you sent me matches the
digest of my internal password, so here is
the document.
Ask user for username and password
digest(”Ow!”)= A3F5
digest(”Ow!”)= A3F5
This is a match!
The Improvements of Digest Authentication |289
What is important about these digests is that if you don’t know the secret password,
you’ll have an awfully hard time guessing the correct digest to send to the server.
And likewise, if you have the digest, you’ll have an awfully hard time figuring out
which of the effectively infinite number of input values generated it.
The 128 bits of MD5 output often are written as 32 hexadecimal characters, each
character representing 4 bits. Table 13-1 shows a few examples of MD5 digests of
sample inputs. Notice how MD5 takes arbitrary inputs and yields a fixed-length
digest output.
Digest functions sometimes are called cryptographic checksums, one-way hash func-
tions, or fingerprint functions.
Using Nonces to Prevent Replays
One-way digests save us from having to send passwords in the clear. We can just
send a digest of the password instead, and rest assured that no malicious party can
easily decode the original password from the digest.
Unfortunately, obscured passwords alone do not save us from danger, because a bad
guy can capture the digest and replay it over and over again to the server, even
though the bad guy doesn’t know the password. The digest is just as good as the
password.
To prevent such replay attacks, the server can pass along to the client a special token
called a nonce,*which changes frequently (perhaps every millisecond, or for every
Table 13-1. MD5 digest examples
Input MD5 digest
“Hi”C1A5298F939E87E8F962A5EDFC206918
“bri:Ow!”BEAAA0E34EBDB072F8627C038AB211F8
“3.1415926535897”475B977E19ECEE70835BC6DF46F4F6DE
“http://www.http-guide.com/index.htm”C617C0C7D1D05F66F595E22A4B0EAAA5
“WE hold these Truths to be self-evident, that all Men are created equal,
that they are endowed by their Creator with certain unalienable Rights,
that among these are Life, Liberty and the Pursuit of Happiness—That to
secure these Rights, Governments are instituted among Men, deriving their
just Powers from the Consent of the Governed, that whenever any Form of
Government becomes destructive of these Ends, it is the Right of the People
to alter or to abolish it, and to institute new Government, laying its Founda-
tion on such Principles, and organizing its Powers in such Form, as to them
shall seem most likely to effect their Safety and Happiness.”
66C4EF58DA7CB956BD04233FBB64E0A4
* The word nonce means “the present occasion” or “the time being.” In a computer-security sense, the nonce
captures a particular point in time and figures that into the security calculations.
290 |Chapter 13: Digest Authentication
authentication). The client appends this nonce token to the password before com-
puting the digest.
Mixing the nonce in with the password causes the digest to change each time the
nonce changes. This prevents replay attacks, because the recorded password digest is
valid only for a particular nonce value, and without the secret password, the attacker
cannot compute the correct digest.
Digest authentication requires the use of nonces, because a trivial replay weakness
would make un-nonced digest authentication effectively as weak as basic authentica-
tion. Nonces are passed from server to client in the WWW-Authenticate challenge.
The Digest Authentication Handshake
The HTTP digest authentication protocol is an enhanced version of authentication
that uses headers similar to those used in basic authentication. Some new options are
added to the traditional headers, and one new optional header, Authorization-Info, is
added.
The simplified three-phase handshake of digest authentication is depicted in
Figure 13-2.
Here’s what’s happening in Figure 13-2:
• In Step 1, the server computes a nonce value. In Step 2, the server sends the
nonce to the client in a WWW-Authenticate challenge message, along with a list
of algorithms that the server supports.
Figure 13-2. Digest authentication handshake
Server
Authorization (response)
Client
WWW-Authenticate (challenge)
(2) Server sends realm, nonce, algorithms
(4) Client sends response digest
[send algorithm]
[send client nonce]
Authentication-Info (info)
(6) Server sends next nonce
[send client rspauth digest]
(1) Server generates nonce
(5) Server verifies digest
[generate rspauth digest]
[generate next nonce]
(3) Choose algorithm from set
[generate response digest]
[generate client-nonce]
(7) Client verifies rspauth digest
Digest Calculations |291
• In Step 3, the client selects an algorithm and computes the digest of the secret
password and the other data. In Step 4, it sends the digest back to the server in
an Authorization message. If the client wants to authenticate the server, it can
send a client nonce.
• In Step 5, the server receives the digest, chosen algorithm, and supporting data
and computes the same digest that the client did. The server then compares the
locally generated digest with the network-transmitted digest and validates that
they match. If the client symmetrically challenged the server with a client nonce,
a client digest is created. Additionally, the next nonce can be precomputed and
handed to the client in advance, so the client can preemptively issue the right
digest the next time.
Many of these pieces of information are optional and have defaults. To clarify things,
Figure 13-3 compares the messages sent for basic authentication (Figure 13-3a–d)
with a simple example of digest authentication (Figure 13-3e–h).
Now let’s look a bit more closely at the internal workings of digest authentication.
Digest Calculations
The heart of digest authentication is the one-way digest of the mix of public informa-
tion, secret information, and a time-limited nonce value. Let’s look now at how the
digests are computed. The digest calculations generally are straightforward.*Sample
source code is provided in Appendix F.
Digest Algorithm Input Data
Digests are computed from three components:
• A pair of functions consisting of a one-way hash function H(d) and digest
KD(s,d), where s stands for secret and d stands for data
• A chunk of data containing security information, including the secret password,
called A1
• A chunk of data containing nonsecret attributes of the request message, called A2
The two pieces of data, A1 and A2, are processed by H and KD to yield a digest.
The Algorithms H(d) and KD(s,d)
Digest authentication supports the selection of a variety of digest algorithms. The
two algorithms suggested in RFC 2617 are MD5 and MD5-sess (where “sess” stands
for session), and the algorithm defaults to MD5 if no other algorithm is specified.
* However, they are made a little more complicated for beginners by the optional compatibility modes of RFC
2617 and by the lack of background material in the specifications. We’ll try to help...
292 |Chapter 13: Digest Authentication
Figure 13-3. Basic versus digest authentication syntax
Client Server
(a) Query
GET /cgi-bin/checkout?cart=17854 HTTP/1.1
Client Server
(b) Challenge
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="Shopping Cart"
Client Server
(c) Response
GET /cgi-bin/checkout?cart=17854 HTTP/1.1
Authorization: Basic YnJpYW4tdG90dHk6T3ch
Client Server
(d) Success
HTTP/1.1 200 OK
...
Basic authentication
Client Server
(e) Query
GET /cgi-bin/checkout?cart=17854 HTTP/1.1
Client Server
(f) Challenge
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest
realm="Shopping Cart"
qop="auth,auth-int"
nonce="66C4EF58DA7CB956BD04233FBB64E0A4"
Digest authentication
Client Server
(g) Response
GET /cgi-bin/checkout?cart=17854 HTTP/1.1
Authorization: Digest
username="bri"
realm="Shopping Cart"
nonce="66C4EF58DA7CB956BD04233FBB64E0A4"
uri="/cgi-bin/checkout?cart=17854"
qop="auth"
nc=0000001,
cnonce="CFA9207102EA210EA210FFC1120F6001110D073"
response="E483C94FOB3CA29109A7BA83D10FE519"
Client Server
(h) Success
HTTP/1.1 200 OK
Authorization-Info: nextnonce=
"29FE72D109C7EF23841AB914F0C3B831"
qop= “auth”
rspauth="89F5A4CE6FA932F6C4DA120CEB754290"
cnonce="CFA9207102EA210EA210FFC1120F6001110D073"
...
Username:
Username:
Password:
Password:
Shopping Cart
Shopping Cart
Digest Calculations |293
If either MD5 or MD5-sess is used, the H function computes the MD5 of the data,
and the KD digest function computes the MD5 of the colon-joined secret and nonse-
cret data. In other words:
H(<data>) = MD5(<data>)
KD(<secret>,<data>) = H(concatenate(<secret>:<data>))
The Security-Related Data (A1)
The chunk of data called A1 is a product of secret and protection information, such
as the username, password, protection realm, and nonces. A1 pertains only to secu-
rity information, not to the underlying message itself. A1 is used along with H, KD,
and A2 to compute digests.
RFC 2617 defines two ways of computing A1, depending on the algorithm chosen:
MD5
One-way hashes are run for every request; A1 is the colon-joined triple of user-
name, realm, and secret password.
MD5-sess
The hash function is run only once, on the first WWW-Authenticate hand-
shake; the CPU-intensive hash of username, realm, and secret password is done
once and prepended to the current nonce and client nonce (cnonce) values.
The definitions of A1 are shown in Table 13-2.
The Message-Related Data (A2)
The chunk of data called A2 represents information about the message itself, such as
the URL, request method, and message entity body. A2 is used to help protect
against method, resource, or message tampering. A2 is used along with H, KD, and
A1 to compute digests.
RFC 2617 defines two schemes for A2, depending on the quality of protection (qop)
chosen:
• The first scheme involves only the HTTP request method and URL. This is used
when qop=“auth”, which is the default case.
• The second scheme adds in the message entity body to provide a degree of mes-
sage integrity checking. This is used when qop=“auth-int”.
Table 13-2. Definitions for A1 by algorithm
Algorithm A1
MD5 A1 = <user>:<realm>:<password>
MD5-sess A1 = MD5(<user>:<realm>:<password>):<nonce>:<cnonce>
294 |Chapter 13: Digest Authentication
The definitions of A2 are shown in Table 13-3.
The request-method is the HTTP request method. The uri-directive-value is the
request URI from the request line. This may be “*,” an “absoluteURL,” or an “abs_
path,” but it must agree with the request URI. In particular, it must be an absolute
URL if the request URI is an absoluteURL.
Overall Digest Algorithm
RFC 2617 defines two ways of computing digests, given H, KD, A1, and A2:
• The first way is intended to be compatible with the older specification RFC
2069, used when the qop option is missing. It computes the digest using the
hash of the secret information and the nonced message data.
• The second way is the modern, preferred approach—it includes support for nonce
counting and symmetric authentication. This approach is used whenever qop is
“auth” or “auth-int”. It adds nonce count, qop, and cnonce data to the digest.
The definitions for the resulting digest function are shown in Table 13-4. Notice the
resulting digests use H, KD, A1, and A2.
It’s a bit easy to get lost in all the layers of derivational encapsulation. This is one of
the reasons that some readers have difficulty with RFC 2617. To try to make it a bit
easier, Table 13-5 expands away the H and KD definitions, and leaves digests in
terms of A1 and A2.
Table 13-3. Definitions for A2 by algorithm (request digests)
qop A2
undefined <request-method>:<uri-directive-value>
auth <request-method>:<uri-directive-value>
auth-int <request-method>:<uri-directive-value>:H(<request-entity-body>)
Table 13-4. Old and new digest algorithms
qop Digest algorithm Notes
undefined KD(H(A1), <nonce>:H(A2)) Deprecated
auth or auth-int KD(H(A1), <nonce>:<nc>:<cnonce>:<qop>:H(A2)) Preferred
Table 13-5. Unfolded digest algorithm cheat sheet
qop Algorithm Unfolded algorithm
undefined <undefined>
MD5
MD5-sess
MD5(MD5(A1):<nonce>:MD5(A2))
Digest Calculations |295
Digest Authentication Session
The client response to a WWW-Authenticate challenge for a protection space starts
an authentication session with that protection space (the realm combined with the
canonical root of the server being accessed defines a “protection space”).
The authentication session lasts until the client receives another WWW-Authenti-
cate challenge from any server in the protection space. A client should remember the
username, password, nonce, nonce count, and opaque values associated with an
authentication session to use to construct the Authorization header in future
requests within that protection space.
When the nonce expires, the server can choose to accept the old Authorization
header information, even though the nonce value included may not be fresh. Alterna-
tively, the server may return a 401 response with a new nonce value, causing the cli-
ent to retry the request; by specifying “stale=true” with this response, the server tells
the client to retry with the new nonce without prompting for a new username and
password.
Preemptive Authorization
In normal authentication, each request requires a request/challenge cycle before the
transaction can be completed. This is depicted in Figure 13-4a.
This request/challenge cycle can be eliminated if the client knows in advance what
the next nonce will be, so it can generate the correct Authorization header before the
server asks for it. If the client can compute the Authorization header before it is
requested, the client can preemptively issue the Authorization header to the server,
without first going through a request/challenge. The performance impact is depicted
in Figure 13-4b.
Preemptive authorization is trivial (and common) for basic authentication. Browsers
commonly maintain client-side databases of usernames and passwords. Once a user
authenticates with a site, the browser commonly sends the correct Authorization
header for subsequent requests to that URL (see Chapter 12).
auth <undefined>
MD5
MD5-sess
MD5(MD5(A1):<nonce>:<nc>:<cnonce>:<qop>:MD5(A2))
auth-int <undefined>
MD5
MD5-sess
MD5(MD5(A1):<nonce>:<nc>:<cnonce>:<qop>:MD5(A2))
Table 13-5. Unfolded digest algorithm cheat sheet (continued)
qop Algorithm Unfolded algorithm
296 |Chapter 13: Digest Authentication
Preemptive authorization is a bit more complicated for digest authentication,
because of the nonce technology intended to foil replay attacks. Because the server
generates arbitrary nonces, there isn’t always a way for the client to determine what
Authorization header to send until it receives a challenge.
Digest authentication offers a few means for preemptive authorization while retain-
ing many of the safety features. Here are three potential ways a client can obtain the
correct nonce without waiting for a new WWW-Authenticate challenge:
• Server pre-sends the next nonce in the Authentication-Info success header.
• Server allows the same nonce to be reused for a small window of time.
• Both the client and server use a synchronized, predictable nonce-generation
algorithm.
Figure 13-4. Preemptive authorization reduces message count
Server
Request
Client
Challenge
Request+authorization
Success
Request
Challenge
Request+authorization
Success
Request
Challenge
Request+authorization
Success
(a) Normal request/challenge
Server
Request
Client
Challenge
Request+authorization
Success+nonceinfo
Request+authorization
Success+nonceinfo
Request+authorization
Success
(b) Preemptive authorization
Digest Calculations |297
Next nonce pregeneration
The next nonce value can be provided in advance to the client by the Authentication-
Info success header. This header is sent along with the 200 OK response from a pre-
vious successful authentication.
Authentication-Info: nextnonce="<nonce-value>"
Given the next nonce, the client can preemptively issue an Authorization header.
While this preemptive authorization avoids a request/challenge cycle (speeding up
the transaction), it also effectively nullifies the ability to pipeline multiple requests to
the same server, because the next nonce value must be received before the next
request can be issued. Because pipelining is expected to be a fundamental technol-
ogy for latency avoidance, the performance penalty may be large.
Limited nonce reuse
Instead of pregenerating a sequence of nonces, another approach is to allow limited
reuse of nonces. For example, a server may allow a nonce to be reused 5 times, or for
10 seconds.
In this case, the client can freely issue requests with the Authorization header, and it
can pipeline them, because the nonce is known in advance. When the nonce finally
expires, the server is expected to send the client a 401 Unauthorized challenge, with
the WWW-Authenticate: stale=true directive set:
WWW-Authenticate: Digest
realm="<realm-value>"
nonce="<nonce-value>"
stale=true
Reusing nonces does reduce security, because it makes it easier for an attacker to
succeed at replay attacks. Because the lifetime of nonce reuse is controllable, from
strictly no reuse to potentially long reuse, trade-offs can be made between windows
of vulnerability and performance.
Additionally, other features can be employed to make replay attacks more difficult,
including incrementing counters and IP address tests. However, while making
attacks more inconvenient, these techniques do not eliminate the vulnerability.
Synchronized nonce generation
It is possible to employ time-synchronized nonce-generation algorithms, where both
the client and the server can generate a sequence of identical nonces, based on a
shared secret key, that a third party cannot easily predict (such as secure ID cards).
These algorithms are beyond the scope of the digest authentication specification.
298 |Chapter 13: Digest Authentication
Nonce Selection
The contents of the nonce are opaque and implementation-dependent. However, the
quality of performance, security, and convenience depends on a smart choice.
RFC 2617 suggests this hypothetical nonce formulation:
BASE64(time-stamp H(time-stamp ":" ETag ":" private-key))
where time-stamp is a server-generated time or other nonrepeating value, ETag is the
value of the HTTP ETag header associated with the requested entity, and private-key
is data known only to the server.
With a nonce of this form, a server will recalculate the hash portion after receiving
the client authentication header and reject the request if it does not match the nonce
from that header or if the time-stamp value is not recent enough. In this way, the
server can limit the duration of the nonce’s validity.
The inclusion of the ETag prevents a replay request for an updated version of the
resource. (Note that including the IP address of the client in the nonce would appear
to offer the server the ability to limit the reuse of the nonce to the same client that orig-
inally got it. However, that would break proxy farms, in which requests from a single
user often go through different proxies. Also, IP address spoofing is not that hard.)
An implementation might choose not to accept a previously used nonce or digest, to
protect against replay attacks. Or, an implementation might choose to use one-time
nonces or digests for POST or PUT requests and time-stamps for GET requests.
Refer to “Security Considerations” for practical security considerations that affect
nonce selection.
Symmetric Authentication
RFC 2617 extends digest authentication to allow the client to authenticate the server.
It does this by providing a client nonce value, to which the server generates a correct
response digest based on correct knowledge of the shared secret information. The
server then returns this digest to the client in the Authorization-Info header.
This symmetric authentication is standard as of RFC 2617. It is optional for back-
ward compatibility with the older RFC 2069 standard, but, because it provides
important security enhancements, all modern clients and servers are strongly recom-
mended to implement all of RFC 2617’s features. In particular, symmetric authenti-
cation is required to be performed whenever a qop directive is present and required
not to be performed when the qop directive is missing.
The response digest is calculated like the request digest, except that the message
body information (A2) is different, because there is no method in a response, and the
message entity data is different. The methods of computation of A2 for request and
response digests are compared in Tables 13-6 and 13-7.
Quality of Protection Enhancements |299
The cnonce value and nc value must be the ones for the client request to which this
message is the response. The response auth, cnonce, and nonce count directives
must be present if qop=“auth” or qop=“auth-int” is specified.
Quality of Protection Enhancements
The qop field may be present in all three digest headers: WWW-Authenticate,
Authorization, and Authentication-Info.
The qop field lets clients and servers negotiate for different types and qualities of pro-
tection. For example, some transactions may want to sanity check the integrity of
message bodies, even if that slows down transmission significantly.
The server first exports a comma-separated list of qop options in the WWW-Authen-
ticate header. The client then selects one of the options that it supports and that
meets its needs and passes it back to the server in its Authorization qop field.
Use of qop is optional, but only for backward compatibility with the older RFC
2069 specification. The qop option should be supported by all modern digest
implementations.
RFC 2617 defines two initial quality of protection values: “auth,” indicating authen-
tication, and “auth-int,” indicating authentication with message integrity protection.
Other qop options are expected in the future.
Message Integrity Protection
If integrity protection is applied (qop=“auth-int”), H (the entity body) is the hash of
the entity body, not the message body. It is computed before any transfer encoding is
applied by the sender and after it has been removed by the recipient. Note that this
includes multipart boundaries and embedded headers in each part of any multipart
content type.
Table 13-6. Definitions for A2 by algorithm (request digests)
qop A2
undefined <request-method>:<uri-directive-value>
auth <request-method>:<uri-directive-value>
auth-int <request-method>:<uri-directive-value>:H(<request-entity-body>)
Table 13-7. Definitions for A2 by algorithm (response digests)
qop A2
undefined :<uri-directive-value>
auth :<uri-directive-value>
auth-int :<uri-directive-value>:H(<response-entity-body>)
300 |Chapter 13: Digest Authentication
Digest Authentication Headers
Both the basic and digest authentication protocols contain an authorization chal-
lenge, carried by the WWW-Authenticate header, and an authorization response,
carried by the Authorization header. Digest authentication adds an optional Authori-
zation-Info header, which is sent after successful authentication, to complete a three-
phase handshake and pass along the next nonce to use. The basic and digest authen-
tication headers are shown in Table 13-8.
The digest authentication headers are quite a bit more complicated. They are
described in detail in Appendix F.
Practical Considerations
There are several things you need to consider when working with digest authentica-
tion. This section discusses some of these issues.
Table 13-8. HTTP authentication headers
Phase Basic Digest
Challenge WWW-Authenticate: Basic
realm="<realm-value>"
WWW-Authenticate: Digest
realm="<realm-value>"
nonce="<nonce-value>"
[domain="<list-of-URIs>"]
[opaque="<opaque-token-value>"]
[stale=<true-or-false>]
[algorithm=<digest-algorithm>]
[qop="<list-of-qop-values>"]
[<extension-directive>]
Response Authorization: Basic
<base64(user:pass)>
Authorization: Digest
username="<username>"
realm="<realm-value>"
nonce="<nonce-value>"
uri=<request-uri>
response="<32-hex-digit-digest>"
[algorithm=<digest-algorithm>]
[opaque="<opaque-token-value>"]
[cnonce="<nonce-value>"]
[qop=<qop-value>]
[nc=<8-hex-digit-nonce-count>]
[<extension-directive>]
Info n/a Authentication-Info:
nextnonce="<nonce-value>"
[qop="<list-of-qop-values>"]
[rspauth="<hex-digest>"]
[cnonce="<nonce-value>"]
[nc=<8-hex-digit-nonce-count>]
Practical Considerations |301
Multiple Challenges
A server can issue multiple challenges for a resource. For example, if a server does
not know the capabilities of a client, it may provide both basic and digest authentica-
tion challenges. When faced with multiple challenges, the client must choose to
answer with the strongest authentication mechanism that it supports.
User agents must take special care in parsing the WWW-Authenticate or Proxy-
Authenticate header field value if it contains more than one challenge or if more than
one WWW-Authenticate header field is provided, as a challenge may itself contain a
comma-separated list of authentication parameters. Note that many browsers recog-
nize only basic authentication and require that it be the first authentication mecha-
nism presented.
There are obvious “weakest link” security concerns when providing a spectrum of
authentication options. Servers should include basic authentication only if it is mini-
mally acceptable, and administrators should caution users about the dangers of shar-
ing the same password across systems when different levels of security are being
employed.
Error Handling
In digest authentication, if a directive or its value is improper, or if a required direc-
tive is missing, the proper response is 400 Bad Request.
If a request’s digest does not match, a login failure should be logged. Repeated fail-
ures from a client may indicate an attacker attempting to guess passwords.
The authenticating server must assure that the resource designated by the “uri” direc-
tive is the same as the resource specified in the request line; if they are different, the
server should return a 400 Bad Request error. (As this may be a symptom of an attack,
server designers may want to consider logging such errors.) Duplicating information
from the request URL in this field deals with the possibility that an intermediate
proxy may alter the client’s request line. This altered (but, presumably, semantically
equivalent) request would not result in the same digest as that calculated by the client.
Protection Spaces
The realm value, in combination with the canonical root URL of the server being
accessed, defines the protection space.
Realms allow the protected resources on a server to be partitioned into a set of pro-
tection spaces, each with its own authentication scheme and/or authorization data-
base. The realm value is a string, generally assigned by the origin server, which may
have additional semantics specific to the authentication scheme. Note that there may
be multiple challenges with the same authorization scheme but different realms.
302 |Chapter 13: Digest Authentication
The protection space determines the domain over which credentials can be automati-
cally applied. If a prior request has been authorized, the same credentials may be
reused for all other requests within that protection space for a period of time deter-
mined by the authentication scheme, parameters, and/or user preference. Unless oth-
erwise defined by the authentication scheme, a single protection space cannot extend
outside the scope of its server.
The specific calculation of protection space depends on the authentication mechanism:
• In basic authentication, clients assume that all paths at or below the request URI
are within the same protection space as the current challenge. A client can pre-
emptively authorize for resources in this space without waiting for another chal-
lenge from the server.
• In digest authentication, the challenge’s WWW-Authenticate: domain field more
precisely defines the protection space. The domain field is a quoted, space-sepa-
rated list of URIs. All the URIs in the domain list, and all URIs logically beneath
these prefixes, are assumed to be in the same protection space. If the domain field
is missing or empty, all URIs on the challenging server are in the protection space.
Rewriting URIs
Proxies may rewrite URIs in ways that change the URI syntax but not the actual
resource being described. For example:
• Hostnames may be normalized or replaced with IP addresses.
• Embedded characters may be replaced with “%” escape forms.
• Additional attributes of a type that doesn’t affect the resource fetched from the
particular origin server may be appended or inserted into the URI.
Because URIs can be changed by proxies, and because digest authentication sanity
checks the integrity of the URI value, the digest authentication will break if any of
these changes are made. See “The Message-Related Data (A2)” for more information.
Caches
When a shared cache receives a request containing an Authorization header and a
response from relaying that request, it must not return that response as a reply to any
other request, unless one of two Cache-Control directives was present in the response:
• If the original response included the “must-revalidate” Cache-Control directive,
the cache may use the entity of that response in replying to a subsequent request.
However, it must first revalidate it with the origin server, using the request head-
ers from the new request, so the origin server can authenticate the new request.
• If the original response included the “public” Cache-Control directive, the
response entity may be returned in reply to any subsequent request.
Security Considerations |303
Security Considerations
RFC 2617 does an admirable job of summarizing some of the security risks inherent
in HTTP authentication schemes. This section describes some of these risks.
Header Tampering
To provide a foolproof system against header tampering, you need either end-to-end
encryption or a digital signature of the headers—preferably a combination of both!
Digest authentication is focused on providing a tamper-proof authentication scheme,
but it does not necessarily extend that protection to the data. The only headers that
have some level of protection are WWW-Authenticate and Authorization.
Replay Attacks
A replay attack, in the current context, is when someone uses a set of snooped
authentication credentials from a given transaction for another transaction. While
this problem is an issue with GET requests, it is vital that a foolproof method for
avoiding replay attacks be available for POST and PUT requests. The ability to suc-
cessfully replay previously used credentials while transporting form data could cause
security nightmares.
Thus, in order for a server to accept “replayed” credentials, the nonce values must be
repeated. One of the ways to mitigate this problem is to have the server generate a
nonce containing a digest of the client’s IP address, a time-stamp, the resource ETag,
and a private server key (as recommended earlier). In such a scenario, the combina-
tion of an IP address and a short timeout value may provide a huge hurdle for the
attacker.
However, this solution has a major drawback. As we discussed earlier, using the cli-
ent’s IP address in creating a nonce breaks transmission through proxy farms, in
which requests from a single user may go through different proxies. Also, IP spoof-
ing is not too difficult.
One way to completely avoid replay attacks is to use a unique nonce value for every
transaction. In this implementation, for each transaction, the server issues a unique
nonce along with a timeout value. The issued nonce value is valid only for the given
transaction, and only for the duration of the timeout value. This accounting may
increase the load on servers; however, the increase should be miniscule.
Multiple Authentication Mechanisms
When a server supports multiple authentication schemes (such as basic and digest),
it usually provides the choice in WWW-Authenticate headers. Because the client is
304 |Chapter 13: Digest Authentication
not required to opt for the strongest authentication mechanism, the strength of the
resulting authentication is only as good as that of the weakest of the authentication
schemes.
The obvious ways to avoid this problem is to have the clients always choose the
strongest authentication scheme available. If this is not practical (as most of us do
use commercially available clients), the only other option is to use a proxy server to
retain only the strongest authentication scheme. However, such an approach is feasi-
ble only in a domain in which all of the clients are known to be able to support the
chosen authentication scheme—e.g., a corporate network.
Dictionary Attacks
Dictionary attacks are typical password-guessing attacks. A malicious user can eaves-
drop on a transaction and use a standard password-guessing program against nonce/
response pairs. If the users are using relatively simple passwords and the servers are
using simplistic nonces, it is quite possible to find a match. If there is no password
aging policy, given enough time and the one-time cost of cracking the passwords, it
is easy to collect enough passwords to do some real damage.
There really is no good way to solve this problem, other than using relatively com-
plex passwords that are hard to crack and a good password aging policy.
Hostile Proxies and Man-in-the-Middle Attacks
Much Internet traffic today goes through a proxy at one point or another. With the
advent of redirection techniques and intercepting proxies, a user may not even real-
ize that his request is going through a proxy. If one of those proxies is hostile or com-
promised, it could leave the client vulnerable to a man-in-the-middle attack.
Such an attack could be in the form of eavesdropping, or altering available authenti-
cation schemes by removing all of the offered choices and replacing them with the
weakest authentication scheme (such as basic authentication).
One of the ways to compromise a trusted proxy is though its extension interfaces.
Proxies sometimes provide sophisticated programming interfaces, and with such
proxies it may be feasible to write an extension (i.e., plug-in) to intercept and modify
the traffic. However, the data-center security and security offered by proxies them-
selves make the possibility of man-in-the-middle attacks via rogue plug-ins quite
remote.
There is no good way to fix this problem. Possible solutions include clients provid-
ing visual cues regarding the authentication strength, configuring clients to always
use the strongest possible authentication, etc., but even when using the strongest
possible authentication scheme, clients still are vulnerable to eavesdropping. The
only foolproof way to guard against these attacks is by using SSL.
Security Considerations |305
Chosen Plaintext Attacks
Clients using digest authentication use a nonce supplied by the server to generate the
response. However, if there is a compromised or malicious proxy in the middle
intercepting the traffic (or a malicious origin server), it can easily supply a nonce for
response computation by the client. Using the known key for computing the
response may make the cryptanalysis of the response easier. This is called a chosen
plaintext attack. There are a few variants of chosen plaintext attacks:
Precomputed dictionary attacks
This is a combination of a dictionary attack and a chosen plaintext attack. First,
the attacking server generates a set of responses, using a predetermined nonce
and common password variations, and creates a dictionary. Once a sizeable dic-
tionary is available, the attacking server/proxy can complete the interdiction of
the traffic and start sending predetermined nonces to the clients. When it gets a
response from a client, the attacker searches the generated dictionary for matches.
If a there is a match, the attacker has the password for that particular user.
Batched brute-force attacks
The difference in a batched brute-force attack is in the computation of the pass-
word. Instead of trying to match a precomputed digest, a set of machines goes to
work on enumerating all of the possible passwords for a given space. As the
machines get faster, the brute-force attack becomes more and more viable.
In general, the threat posed by these attacks is easily countered. One way to prevent
them is to configure clients to use the optional cnonce directive, so that the response
is generated at the client’s discretion, not using the nonce supplied by the server
(which could be compromised by the attacker). This, combined with policies enforc-
ing reasonably strong passwords and a good password aging mechanism, can miti-
gate the threat of chosen plaintext attacks completely.
Storing Passwords
The digest authentication mechanism compares the user response to what is stored
internally by the server—usually, usernames and H(A1) tuples, where H(A1) is
derived from the digest of username, realm, and password.
Unlike with a traditional password file on a Unix box, if a digest authentication pass-
word file is compromised, all of the documents in the realm immediately are avail-
able to the attacker; there is no need for a decrypting step.
Some of the ways to mitigate this problem are to:
• Protect the password file as though it contained clear-text passwords.
• Make sure the realm name is unique among all the realms, so that if a password
file is compromised, the damage is localized to a particular realm. A fully quali-
fied realm name with host and domain included should satisfy this requirement.
306 |Chapter 13: Digest Authentication
While digest authentication provides a much more robust and secure solution than
basic authentication, it still does not provide any protection for security of the con-
tent—a truly secure transaction is feasible only through SSL, which we describe in
the next chapter.
For More Information
For more information on authentication, see:
http://www.ietf.org/rfc/rfc2617.txt
RFC 2617, “HTTP Authentication: Basic and Digest Access Authentication.”
307
CHAPTER 14
Secure HTTP
The previous three chapters reviewed features of HTTP that help identify and
authenticate users. These techniques work well in a friendly community, but they
aren’t strong enough to protect important transactions from a community of moti-
vated and hostile adversaries.
This chapter presents a more complicated and aggressive technology to secure HTTP
transactions from eavesdropping and tampering, using digital cryptography.
Making HTTP Safe
People use web transactions for serious things. Without strong security, people
wouldn’t feel comfortable doing online shopping and banking. Without being able
to restrict access, companies couldn’t place important documents on web servers.
The Web requires a secure form of HTTP.
The previous chapters talked about some lightweight ways of providing authentica-
tion (basic and digest authentication) and message integrity (digest qop=“auth-int”).
These schemes are good for many purposes, but they may not be strong enough for
large purchases, bank transactions, or access to confidential data. For these more
serious transactions, we combine HTTP with digital encryption technology.
A secure version of HTTP needs to be efficient, portable, easy to administer, and
adaptable to the changing world. It also has to meet societal and governmental
requirements. We need a technology for HTTP security that provides:
• Server authentication (clients know they’re talking to the real server, not a phony)
• Client authentication (servers know they’re talking to the real user, not a phony)
• Integrity (clients and servers are safe from their data being changed)
• Encryption (clients and servers talk privately without fear of eavesdropping)
• Efficiency (an algorithm fast enough for inexpensive clients and servers to use)
• Ubiquity (protocols are supported by virtually all clients and servers)
308 |Chapter 14: Secure HTTP
• Administrative scalability (instant secure communication for anyone, anywhere)
• Adaptability (supports the best known security methods of the day)
• Social viability (meets the cultural and political needs of the society)
HTTPS
HTTPS is the most popular secure form of HTTP. It was pioneered by Netscape
Communications Corporation and is supported by all major browsers and servers.
You can tell if a web page was accessed through HTTPS instead of HTTP, because
the URL will start with the scheme https:// instead of http:// (some browsers also dis-
play iconic security cues, as shown in Figure 14-1).
When using HTTPS, all the HTTP request and response data is encrypted before
being sent across the network. HTTPS works by providing a transport-level crypto-
graphic security layer—using either the Secure Sockets Layer (SSL) or its successor,
Transport Layer Security (TLS)—underneath HTTP (Figure 14-2). Because SSL and
TLS are so similar, in this book we use the term “SSL” loosely to represent both SSL
and TLS.
Because most of the hard encoding and decoding work happens in the SSL libraries,
web clients and servers don’t need to change much of their protocol processing logic
Figure 14-1. Browsing secure web sites
https scheme
security icon
Digital Cryptography |309
to use secure HTTP. For the most part, they simply need to replace TCP input/out-
put calls with SSL calls and add a few other calls to configure and manage the secu-
rity information.
Digital Cryptography
Before we talk in detail about HTTPS, we need to provide a little background about
the cryptographic encoding techniques used by SSL and HTTPS. In the next few sec-
tions, we’ll give a speedy primer of the essentials of digital cryptography. If you
already are familiar with the technology and terminology of digital cryptography, feel
free to jump ahead to “HTTPS: The Details.”
In this digital cryptography primer, we’ll talk about:
Ciphers
Algorithms for encoding text to make it unreadable to voyeurs
Keys
Numeric parameters that change the behavior of ciphers
Symmetric-key cryptosystems
Algorithms that use the same key for encoding and decoding
Asymmetric-key cryptosystems
Algorithms that use different keys for encoding and decoding
Public-key cryptography
A system making it easy for millions of computers to send secret messages
Digital signatures
Checksums that verify that a message has not been forged or tampered with
Digital certificates
Identifying information, verified and signed by a trusted organization
Figure 14-2. HTTPS is HTTP layered over a security layer, layered over TCP
HTTP Application layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(a) HTTP
HTTP Application layer
SSL or TLS Security layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(b) HTTPS
310 |Chapter 14: Secure HTTP
The Art and Science of Secret Coding
Cryptography is the art and science of encoding and decoding messages. People have
used cryptographic methods to send secret messages for thousands of years. How-
ever, cryptography can do more than just encrypt messages to prevent reading by
nosy folks; it also can be used to prevent tampering with messages. Cryptography
even can be used to prove that you indeed authored a message or transaction, just
like your handwritten signature on a check or an embossed wax seal on an envelope.
Ciphers
Cryptography is based on secret codes called ciphers. A cipher is a coding scheme—a
particular way to encode a message and an accompanying way to decode the secret
later. The original message, before it is encoded, often is called plaintext or cleartext.
The coded message, after the cipher is applied, often is called ciphertext. Figure 14-3
shows a simple example.
Ciphers have been used to generate secret messages for thousands of years. Legend has
it that Julius Caesar used a three-character rotation cipher, where each character in the
message is replaced with a character three alphabetic positions forward. In our mod-
ern alphabet, “A” would be replaced by “D,” “B” would be replaced by “E,” and so on.
For example, in Figure 14-4, the message “meet me at the pier at midnight” encodes
into the ciphertext “phhw ph dw wkh slhu dw plgqljkw” using the rot3 (rotate by 3
characters) cipher.*The ciphertext can be decrypted back to the original plaintext
message by applying the inverse coding, rotating –3 characters in the alphabet.
Figure 14-3. Plaintext and ciphertext
* For simplicity of example, we aren’t rotating punctuation or whitespace, but you could.
Figure 14-4. Rotate-by-3 cipher example
Plaintext
Meet me at the pier
at midnight
Encoder
Ciphertext
Phhw ph dw wkh slhu
dw plgqljkw
Decoder
Plaintext
Meet me at the pier
at midnight
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZABC
Cipher
Plaintext MEET
Ciphertext PHHW
ME
PH
AT
DW
THE
WKH
PIER
SLHU
AT
DW
MIDNIGHT
PLGQLJKWDW
AT
Digital Cryptography |311
Cipher Machines
Ciphers began as relatively simple algorithms, because human beings needed to do
the encoding and decoding themselves. Because the ciphers were simple, people
could work the codes using pencil and paper and code books. However, it also was
possible for clever people to “crack” the codes fairly easily.
As technology advanced, people started making machines that could quickly and
accurately encode and decode messages using much more complicated ciphers.
Instead of just doing simple rotations, these cipher machines could substitute charac-
ters, transpose the order of characters, and slice and dice messages to make codes
much harder to crack.*
Keyed Ciphers
Because code algorithms and machines could fall into enemy hands, most machines
had dials that could be set to a large number of different values that changed how the
cipher worked. Even if the machine was stolen, without the right dial settings (key
values) the decoder wouldn’t work.†
These cipher parameters were called keys. You needed to enter the right key into the
cipher machine to get the decoding process to work correctly. Cipher keys make a
single cipher machine act like a set of many virtual cipher machines, each of which
behaves differently because they have different key values.
Figure 14-5 illustrates an example of keyed ciphers. The cipher algorithm is the triv-
ial “rotate-by-N” cipher. The value of N is controlled by the key. The same input
message, “meet me at the pier at midnight,” passed through the same encoding
machine, generates different outputs depending on the value of the key. Today, vir-
tually all cipher algorithms use keys.
Digital Ciphers
With the advent of digital computation, two major advances occurred:
• Complicated encoding and decoding algorithms became possible, freed from the
speed and function limitations of mechanical machinery.
* Perhaps the most famous mechanical code machine was the World War II German Enigma code machine.
Despite the complexity of the Enigma cipher, Alan Turing and colleagues were able to crack the Enigma
codes in the early 1940s, using the earliest digital computers.
† In reality, having the logic of the machine in your possession can sometimes help you to crack the code,
because the machine logic may point to patterns that you can exploit. Modern cryptographic algorithms usu-
ally are designed so that even if the algorithm is publicly known, it’s difficult to come up with any patterns
that will help evildoers crack the code. In fact, many of the strongest ciphers in common use have their
source code available in the public domain, for all to see and study!
312 |Chapter 14: Secure HTTP
• It became possible to support very large keys, so that a single cipher algorithm
could yield trillions of virtual cipher algorithms, each differing by the value of
the key. The longer the key, the more combinations of encodings are possible,
and the harder it is to crack the code by randomly guessing keys.
Unlike physical metal keys or dial settings in mechanical devices, digital keys are just
numbers. These digital key values are inputs to the encoding and decoding algo-
rithms. The coding algorithms are functions that take a chunk of data and encode/
decode it based on the algorithm and the value of the key.
Given a plaintext message called P, an encoding function called E, and a digital
encoding key called e, you can generate a coded ciphertext message C (Figure 14-6).
You can decode the ciphertext C back into the original plaintext P by using the
decoder function D and the decoding key d. Of course, the decoding and encoding
functions are inverses of each other; the decoding of the encoding of P gives back the
original message P.
Figure 14-5. The rotate-by-N cipher, using different keys
Plaintext
Meet me at the pier
at midnight
Rotate(n) encoder
Ciphertext
nffu nf bu uif qjfs
bu njeojhiu
Key= 1
(a)
Plaintext
Meet me at the pier
at midnight
Rotate(n) encoder
Ciphertext
oggv og cv vjg
rkgt cv okfpkijv
Key= 2
(b)
Plaintext
Meet me at the pier
at midnight
Rotate(n) encoder
Ciphertext
phhw ph dw wkh
slhu dw plgqlijkw
Key= 3
(c)
Symmetric-Key Cryptography |313
Symmetric-Key Cryptography
Let’s talk in more detail about how keys and ciphers work together. Many digital
cipher algorithms are called symmetric-key ciphers, because they use the same key
value for encoding as they do for decoding (e = d). Let’s just call the key k.
In a symmetric key cipher, both a sender and a receiver need to have the same shared
secret key, k, to communicate. The sender uses the shared secret key to encrypt the
message and sends the resulting ciphertext to the receiver. The receiver takes the
ciphertext and applies the decrypting function, along with the same shared secret
key, to recover the original plaintext (Figure 14-7).
Some popular symmetric-key cipher algorithms are DES, Triple-DES, RC2, and RC4.
Key Length and Enumeration Attacks
It’s very important that secret keys stay secret. In most cases, the encoding and
decoding algorithms are public knowledge, so the key is the only thing that’s secret!
A good cipher algorithm forces the enemy to try every single possible key value in the
universe to crack the code. Trying all key values by brute force is called an enumera-
tion attack. If there are only a few possible key values, a bad guy can go through all of
them by brute force and eventually crack the code. But if there are a lot of possible
key values, it might take the bad guy days, years, or even the lifetime of the universe
to go through all the keys, looking for one that breaks the cipher.
Figure 14-6. Plaintext is encoded with encoding key e, and decoded using decoding key d
Figure 14-7. Symmetric-key cryptography algorithms use the same key for encoding and decoding
Plaintext P
Encoder E
Ciphertext C
Key= e
C = E(P, e)
P = D(C, d)
Plaintext P
Decoder D
Ciphertext C
Key= d
314 |Chapter 14: Secure HTTP
The number of possible key values depends on the number of bits in the key and how
many of the possible keys are valid. For symmetric-key ciphers, usually all of the key
values are valid.*An 8-bit key would have only 256 possible keys, a 40-bit key would
have 240 possible keys (around one trillion keys), and a 128-bit key would generate
around 340,000,000,000,000,000,000,000,000,000,000,000,000 possible keys.
For conventional symmetric-key ciphers, 40-bit keys are considered safe enough for
small, noncritical transactions. However, they are breakable by today’s high-speed
workstations, which can now do billions of calculations per second.
In contrast, 128-bit keys are considered very strong for symmetric-key cryptography.
In fact, long keys have such an impact on cryptographic security that the U.S. gov-
ernment has put export controls on cryptographic software that uses long keys, to
prevent potentially antagonistic organizations from creating secret codes that the U.
S. National Security Agency (NSA) would itself be unable to crack.
Bruce Schneier’s excellent book, Applied Cryptography (John Wiley & Sons),
includes a table describing the time it would take to crack a DES cipher by guessing
all keys, using 1995 technology and economics.†Excerpts of this table are shown in
Table 14-1.
Given the speed of 1995 microprocessors, an attacker willing to spend $100,000 in
1995 could break a 40-bit DES code in about 2 seconds. And computers in 2002
already are 20 times faster than they were in 1995. Unless the users change keys fre-
quently, 40-bit keys are not safe against motivated opponents.
The DES standard key size of 56 bits is more secure. In 1995 economics, a $1 mil-
lion assault still would take several hours to crack the code. But a person with access
to supercomputers could crack the code by brute force in a matter of seconds. In
* There are ciphers where only some of the key values are valid. For example, in RSA, the best-known
asymmetric-key cryptosystem, valid keys must be related to prime numbers in a certain way. Only a small
number of the possible key values have this property.
† Computation speed has increased dramatically since 1995, and cost has been reduced. And the longer it
takes you to read this book, the faster they’ll become! However, the table still is relatively useful, even if the
times are off by a factor of 5, 10, or more.
Table 14-1. Longer keys take more effort to crack (1995 data, from “Applied Cryptography”)
Attack cost 40-bit key 56-bit key 64-bit key 80-bit key 128-bit key
$100,000 2 secs 35 hours 1 year 70,000 years 1019 years
$1,000,000 200 msecs 3.5 hours 37 days 7,000 years 1018 years
$10,000,000 20 msecs 21 mins 4 days 700 years 1017 years
$100,000,000 2 msecs 2 mins 9 hours 70 years 1016 years
$1,000,000,000 200 usecs 13 secs 1 hour 7 years 1015 years
Public-Key Cryptography |315
contrast, 128-bit DES keys, similar in size to Triple-DES keys, are believed to be
effectively unbreakable by anyone, at any cost, using a brute-force attack.*
Establishing Shared Keys
One disadvantage of symmetric-key ciphers is that both the sender and receiver have
to have a shared secret key before they can talk to each other.
If you wanted to talk securely with Joe’s Hardware store, perhaps to order some wood-
working tools after watching a home-improvement program on public television,
you’d have to establish a private secret key between you and www.joes-hardware.com
before you could order anything securely. You’d need a way to generate the secret key
and to remember it. Both you and Joe’s Hardware, and every other Internet user,
would have thousands of keys to generate and remember.
Say that Alice (A), Bob (B), and Chris (C) all wanted to talk to Joe’s Hardware (J). A,
B, and C each would need to establish their own secret keys with J. A would need
key kAJ, B would need key kBJ, and C would need key kCJ. Every pair of communicat-
ing parties needs its own private key. If there are N nodes, and each node has to talk
securely with all the other N–1 nodes, there are roughly N2total secret keys: an
administrative nightmare.
Public-Key Cryptography
Instead of a single encoding/decoding key for every pair of hosts, public-key cryptog-
raphy uses two asymmetric keys: one for encoding messages for a host, and another
for decoding the host’s messages. The encoding key is publicly known to the world
(thus the name public-key cryptography), but only the host knows the private decod-
ing key (see Figure 14-8). This makes key establishment much easier, because every-
one can find the public key for a particular host. But the decoding key is kept secret,
so only the recipient can decode messages sent to it.
Node X can take its encoding key exand publish it publicly.†Now anyone wanting
to send a message to node X can use the same, well-known public key. Because each
host is assigned an encoding key, which everyone uses, public-key cryptography
avoids the N2explosion of pairwise symmetric keys (see Figure 14-9).
* A large key does not mean that the cipher is foolproof, though! There may be an unnoticed flaw in the cipher
algorithm or implementation that provides a weakness for an attacker to exploit. It’s also possible that the
attacker may have some information about how the keys are generated, so that he knows some keys are more
likely than others, helping to focus a brute-force attack. Or a user might leave the secret key someplace where
an attacker might be able to steal it.
† As we’ll see later, most public-key lookup actually is done through digital certificates, but the details of how
you find public keys don’t matter much now—just know that they are publicly available somewhere.
316 |Chapter 14: Secure HTTP
Even though everyone can encode messages to X with the same key, no one other
than X can decode the messages, because only X has the decoding private key dx.
Splitting the keys lets anyone encode a message but restricts the ability to decode
messages to only the owner. This makes it easier for nodes to securely send mes-
sages to servers, because they can just look up the server’s public key.
Public-key encryption technology makes it possible to deploy security protocols to
every computer user around the world. Because of the great importance of making a
Figure 14-8. Public-key cryptography is asymmetric, using different keys for encoding and decoding
Figure 14-9. Public-key cryptography assigns a single, public encoding key to each host
Plaintext
Private
key= ds
Plaintext
Encrypted ciphertext
Public
key= es
Server
Client
Internet
kAX
kCX
kBX kDX
ex
ex
ex ex
(a) Symmetric-key cryptography (b) Public-key cryptography
C
BD
A
C
BD
A
X X
Digital Signatures |317
standardized public-key technology suite, a massive Public-Key Infrastructure (PKI)
standards initiative has been under way for well over a decade.
RSA
The challenge of any public-key asymmetric cryptosystem is to make sure no bad guy
can compute the secret, private key—even if he has all of the following clues:
• The public key (which anyone can get, because it’s public)
• A piece of intercepted ciphertext (obtained by snooping the network)
• A message and its associated ciphertext (obtained by running the encoder on any
text)
One popular public-key cryptosystem that meets all these needs is the RSA algo-
rithm, invented at MIT and subsequently commercialized by RSA Data Security.
Given a public key, an arbitrary piece of plaintext, the associated ciphertext from
encoding the plaintext with the public key, the RSA algorithm itself, and even the
source code of the RSA implementation, cracking the code to find the corresponding
private key is believed to be as hard a problem as computing huge prime numbers—
believed to be one of the hardest problems in all of computer science. So, if you can
find a fast way of factoring large numbers into primes, not only can you break into
Swiss bank accounts, but you can also win a Turing Award.
The details of RSA cryptography involve some tricky mathematics, so we won’t go
into them here. There are plenty of libraries available to let you perform the RSA
algorithms without you needing a Ph.D. in number theory.
Hybrid Cryptosystems and Session Keys
Asymmetric, public-key cryptography is nifty, because anyone can send secure mes-
sages to a public server, just by knowing its public key. Two nodes don’t first have to
negotiate a private key in order to communicate securely.
But public-key cryptography algorithms tend to be computationally slow. In prac-
tice, mixtures of both symmetric and asymmetric schemes are used. For example, it
is common to use public-key cryptography to conveniently set up secure communi-
cation between nodes but then to use that secure channel to generate and communi-
cate a temporary, random symmetric key to encrypt the rest of the data through
faster, symmetric cryptography.
Digital Signatures
So far, we’ve been talking about various kinds of keyed ciphers, using symmetric and
asymmetric keys, to allow us to encrypt and decrypt secret messages.
318 |Chapter 14: Secure HTTP
In addition to encrypting and decrypting messages, cryptosystems can be used to
sign messages, proving who wrote the message and proving the message hasn’t been
tampered with. This technique, called digital signing, is important for Internet secu-
rity certificates, which we discuss in the next section.
Signatures Are Cryptographic Checksums
Digital signatures are special cryptographic checksums attached to a message. They
have two benefits:
• Signatures prove the author wrote the message. Because only the author has the
author’s top-secret private key,*only the author can compute these checksums.
The checksum acts as a personal “signature” from the author.
• Signatures prevent message tampering. If a malicious assailant modified the mes-
sage in-flight, the checksum would no longer match. And because the checksum
involves the author’s secret, private key, the intruder will not be able to fabricate
a correct checksum for the tampered-with message.
Digital signatures often are generated using asymmetric, public-key technology. The
author’s private key is used as a kind of “thumbprint,” because the private key is
known only by the owner.
Figure 14-10 shows an example of how node A can send a message to node B and
sign it:
• Node A distills the variable-length message into a fixed-sized digest.
• Node A applies a “signature” function to the digest that uses the user’s private
key as a parameter. Because only the user knows the private key, a correct signa-
ture function shows the signer is the owner. In Figure 14-10, we use the decoder
function D as the signature function, because it involves the user’s private key.†
• Once the signature is computed, node A appends it to the end of the message
and sends both the message and the signature to node B.
• On receipt, if node B wants to make sure that node A really wrote the message,
and that the message hasn’t been tampered with, node B can check the signa-
ture. Node B takes the private-key scrambled signature and applies the inverse
function using the public key. If the unpacked digest doesn’t match node B’s
own version of the digest, either the message was tampered with in-flight, or the
sender did not have node A’s private key (and therefore was not node A).
* This assumes the private key has not been stolen. Most private keys expire after a while. There also are “revo-
cation lists” that keep track of stolen or compromised keys.
† With the RSA cryptosystem, the decoder function D is used as the signature function, because D already
takes the private key as input. Note that the decoder function is just a function, so it can be used on any
input. Also, in the RSA cryptosystem, the D and E functions work when applied in either order and cancel
each other out. So, E(D(stuff)) = stuff, just as D(E(stuff)) = stuff.
Digital Certificates |319
Digital Certificates
In this section, we talk about digital certificates, the “ID cards” of the Internet. Digi-
tal certificates (often called “certs,” like the breath mints) contain information about
a user or firm that has been vouched for by a trusted organization.
We all carry many forms of identification. Some IDs, such as passports and drivers’
licenses, are trusted enough to prove one’s identity in many situations. For example,
a U.S. driver’s license is sufficient proof of identity to let you board an airplane to
New York for New Year’s Eve, and it’s sufficient proof of your age to let you drink
intoxicating beverages with your friends when you get there.
More trusted forms of identification, such as passports, are signed and stamped by a
government on special paper. They are harder to forge, so they inherently carry a
higher level of trust. Some corporate badges and smart cards include electronics to
help strengthen the identity of the carrier. Some top-secret government organiza-
tions even need to match up your fingerprints or retinal capillary patterns with your
ID before trusting it!
Other forms of ID, such as business cards, are relatively easy to forge, so people trust
this information less. They may be fine for professional interactions but probably are
not enough proof of employment when you apply for a home loan.
The Guts of a Certificate
Digital certificates also contain a set of information, all of which is digitally signed by
an official “certificate authority.” Basic digital certificates commonly contain basic
things common to printed IDs, such as:
• Subject’s name (person, server, organization, etc.)
• Expiration date
Figure 14-10. Unencrypted digital signature
A
Private
key= dA
Message
digest
DSignature
Plaintext
message
Public
key= eA
E
Message digest
B
Same?
Message
digest
320 |Chapter 14: Secure HTTP
• Certificate issuer (who is vouching for the certificate)
• Digital signature from the certificate issuer
Additionally, digital certificates often contain the public key of the subject, as well as
descriptive information about the subject and about the signature algorithm used.
Anyone can create a digital certificate, but not everyone can get a well-respected sign-
ing authority to vouch for the certificate’s information and sign the certificate with
its private key. A typical certificate structure is shown in Figure 14-11.
X.509 v3 Certificates
Unfortunately, there is no single, universal standard for digital certificates. There are
many, subtly different styles of digital certificates, just as not all printed ID cards con-
tain the same information in the same place. The good news is that most certificates
in use today store their information in a standard form, called X.509 v3. X.509 v3 cer-
tificates provide a standard way of structuring certificate information into parseable
fields. Different kinds of certificates have different field values, but most follow the
X.509 v3 structure. The fields of an X.509 certificate are described in Table 14-2.
Figure 14-11. Typical digital signature format
Table 14-2. X.509 certificate fields
Field Description
Version The X.509 certificate version number for this certificate. Usually version 3 today.
Serial Number A unique integer generated by the certification authority. Each certificate from a CA must
have a unique serial number.
Signature Algorithm ID The cryptographic algorithm used for the signature. For example, “MD2 digest with RSA
encryption”.
Certificate Issuer The name for the organization that issued and signed this certificate, in X.500 format.
Validity Period When this certificate is valid, defined by a start date and an end date.
Certificate format version number
Digital signature
function
Certificate serial number
Certificate signature algorithm
Certificate issuer
Validity period
Subject’s name
Subject’s public key
Other extension information
Digital signature
Digital Certificates |321
There are several flavors of X.509-based certificates, including (among others) web
server certificates, client email certificates, software code-signing certificates, and cer-
tificate authority certificates.
Using Certificates to Authenticate Servers
When you establish a secure web transaction through HTTPS, modern browsers
automatically fetch the digital certificate for the server being connected to. If the
server does not have a certificate, the secure connection fails. The server certificate
contains many fields, including:
• Name and hostname of the web site
• Public key of the web site
• Name of the signing authority
• Signature from the signing authority
When the browser receives the certificate, it checks the signing authority.*If it is a
public, well-respected signing authority, the browser will already know its public key
Subject’s Name The entity described in the certificate, such as a person or an organization. The subject
name is in X.500 format.
Subject’s Public Key Information The public key for the certificate’s subject, the algorithm used for the public key, and any
additional parameters.
Issuer Unique ID (optional) An optional unique identifier for the certificate issuer, to allow the potential reuse of the
same issuer name.
Subject Unique ID (optional) An optional unique identifier for the certificate subject, toallow the potential reuseof the
same subject name.
Extensions An optional set of extension fields (in version 3 and higher). Each extension field is flagged
as critical or noncritical. Critical extensions are important and must be understood by the
certificate user. If a certificate user doesn’t recognize a critical extension field, it must
reject the certificate. Common extension fields in use include:
Basic Constraints
Subject’s relationship to certification authority
Certificate Policy
The policy under which the certificate is granted
Key Usage
Restricts how the public key can be used
Certification Authority Signature The certification authority’s digital signature of all of the above fields, using the specified
signing algorithm.
* Browsers and other Internet applications try hard to hide the details of most certificate management, to make
browsing easier. But, when you are browsing through secure connections, all the major browsers allow you
to personally examine the certificates of the sites to which you are talking, to be sure all is on the up-and-up.
Table 14-2. X.509 certificate fields (continued)
Field Description
322 |Chapter 14: Secure HTTP
(browsers ship with certificates of many signing authorities preinstalled), so it can
verify the signature as we discussed in the previous section, “Digital Signatures.”
Figure 14-12 shows how a certificate’s integrity is verified using its digital signature.
If the signing authority is unknown, the browser isn’t sure if it should trust the sign-
ing authority and usually displays a dialog box for the user to read and see if he trusts
the signer. The signer might be the local IT department, or a software vendor.
HTTPS: The Details
HTTPS is the most popular secure version of HTTP. It is widely implemented and
available in all major commercial browsers and servers. HTTPS combines the HTTP
protocol with a powerful set of symmetric, asymmetric, and certificate-based crypto-
graphic techniques, making HTTPS very secure but also very flexible and easy to
administer across the anarchy of the decentralized, global Internet.
HTTPS has accelerated the growth of Internet applications and has been a major
force in the rapid growth of web-based electronic commerce. HTTPS also has been
critical in the wide-area, secure administration of distributed web applications.
HTTPS Overview
HTTPS is just HTTP sent over a secure transport layer. Instead of sending HTTP
messages unencrypted to TCP and across the world-wide Internet (Figure 14-13a),
HTTPS sends the HTTP messages first to a security layer that encrypts them before
sending them to TCP (Figure 14-13b).
Figure 14-12. Verifying that a signature is real
Certificate format version number
Certificate serial number
Certificate signature algorithm
Certificate issuer
(signing authority)
Validity period
Subject’s name
Subject’s public key
Other extension information
Digital signature
B
Signing authority’s
public key
Message digest
E
Message
digest
Same?
HTTPS: The Details |323
Today, the HTTP security layer is implemented by SSL and its modern replacement,
TLS. We follow the common practice of using the term “SSL” to mean either SSL or
TLS.
HTTPS Schemes
Today, secure HTTP is optional. Thus, when making a request to a web server, we
need a way to tell the web server to perform the secure protocol version of HTTP.
This is done in the scheme of the URL.
In normal, nonsecure HTTP, the scheme prefix of the URL is http, as in:
http://www.joes-hardware.com/index.html
In the secure HTTPS protocol, the scheme prefix of the URL is https, as in:
https://cajun-shop.securesites.com/Merchant2/merchant.mv?Store_Code=AGCGS
When a client (such as a web browser) is asked to perform a transaction on a web
resource, it examines the scheme of the URL:
• If the URL has an http scheme, the client opens a connection to the server on
port 80 (by default) and sends it plain-old HTTP commands (Figure 14-14a).
• If the URL has an https scheme, the client opens a connection to the server on
port 443 (by default) and then “handshakes” with the server, exchanging some
SSL security parameters with the server in a binary format, followed by the
encrypted HTTP commands (Figure 14-14b).
Because SSL traffic is a binary protocol, completely different from HTTP, the traffic
is carried on different ports (SSL usually is carried over port 443). If both SSL and
HTTP traffic arrived on port 80, most web servers would interpret binary SSL traffic
as erroneous HTTP and close the connection. A more integrated layering of security
services into HTTP would have eliminated the need for multiple destination ports,
but this does not cause severe problems in practice.
Let’s look a bit more closely at how SSL sets up connections with secure servers.
Figure 14-13. HTTP transport-level security
HTTP Application layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(a) HTTP
HTTP Application layer
SSL or TLS Security layer
TCP Transport layer
IP Network layer
Network interfaces Data link layer
(b) HTTPS
324 |Chapter 14: Secure HTTP
Secure Transport Setup
In unencrypted HTTP, a client opens a TCP connection to port 80 on a web server,
sends a request message, receives a response message, and closes the connection.
This sequence is sketched in Figure 14-15a.
The procedure is slightly more complicated in HTTPS, because of the SSL security
layer. In HTTPS, the client first opens a connection to port 443 (the default port for
secure HTTP) on the web server. Once the TCP connection is established, the client
and server initialize the SSL layer, negotiating cryptography parameters and exchang-
ing keys. When the handshake completes, the SSL initialization is done, and the cli-
ent can send request messages to the security layer. These messages are encrypted
before being sent to TCP. This procedure is depicted in Figure 14-15b.
SSL Handshake
Before you can send encrypted HTTP messages, the client and server need to do an
SSL handshake, where they:
• Exchange protocol version numbers
• Select a cipher that each side knows
• Authenticate the identity of each side
• Generate temporary session keys to encrypt the channel
Figure 14-14. HTTP and HTTPS port numbers
Client Server
HTTP
80
(a) HTTP request
Client Secure server
HTTPS
443
(b) HTTPS request
Client
Proxy
HTTPS tunnel
8080
(c) HTTPS over HTTP tunnel
Secure server
HTTPS
443
HTTPS: The Details |325
Before any encrypted HTTP data flies across the network, SSL already has sent a
bunch of handshake data to establish the communication. The essence of the SSL
handshake is shown in Figure 14-16.
This is a simplified version of the SSL handshake. Depending on how SSL is being
used, the handshake can be more complicated, but this is the general idea.
Figure 14-15. HTTP and HTTPS transactions
E D
ED
Client Server
Establish TCP connection to server port 443
Client Server
SSL security parameters handshake
Client Server
Internet
HTTP request sent over SSL/encrypted request sent over TCP
Client Server
Internet
HTTP response sent over SSL/encrypted response sent over TCP
Internet
Internet
Client Server
SSL close notification
Internet
Client Server
TCP connection close
Internet
Client Server
Establish TCP connection to server port 80
Client Server
Internet
HTTP request sent over TCP
Client Server
Internet
HTTP response sent over TCP
Internet
Client Server
TCP connection close
Internet
80 443
1
2
3
4
5
6
(a) Unencrypted HTTP transaction (b) Encrypted HTTPS transaction
326 |Chapter 14: Secure HTTP
Server Certificates
SSL supports mutual authentication, carrying server certificates to clients and carry-
ing client certificates back to servers. But today, client certificates are not commonly
used for browsing. Most users don’t even possess personal client certificates.*A web
server can demand a client certificate, but that seldom occurs in practice.†
On the other hand, secure HTTPS transactions always require server certificates.
When you perform a secure transaction on a web server, such as posting your credit
card information, you want to know that you are talking to the organization you
think you are talking to. Server certificates, signed by a well-known authority, help
you assess how much you trust the server before sending your credit card or per-
sonal information.
The server certificate is an X.509 v3–derived certificate showing the organization’s
name, address, server DNS domain name, and other information (see Figure 14-17).
You and your client software can examine the certificate to make sure everything
seems to be on the up-and-up.
Figure 14-16. SSL handshake (simplified)
* Client certificates are used for web browsing in some corporate settings, and client certificates are used for
secure email. In the future, client certificates may become more common for web browsing, but today
they’ve caught on very slowly.
† Some organizational intranets use client certificates to control employee access to information.
Server
certificate
Server
SSL security parameters handshake
Client Server
(1) Client sends cipher choices and requests certification
Internet
Client Server
(2) Server sends chosen cipher and certificate
Internet
Client Server
(3) Client sends secret; client and server make keys
Internet
Client Server
(4) Client and server tell each other to start encryption
Internet
Client ServerInternet
HTTPS: The Details |327
Site Certificate Validation
SSL itself doesn’t require you to examine the web server certificate, but most mod-
ern browsers do some simple sanity checks on certificates and provide you with the
means to do more thorough checks. One algorithm for web server certificate valida-
tion, proposed by Netscape, forms the basis of most browser’s validation tech-
niques. The steps are:
Date check
First, the browser checks the certificate’s start and end dates to ensure the certifi-
cate is still valid. If the certificate has expired or has not yet become active, the
certificate validation fails and the browser displays an error.
Signer trust check
Every certificate is signed by some certificate authority (CA), who vouches for
the server. There are different levels of certificate, each requiring different levels
of background verification. For example, if you apply for an e-commerce server
certificate, you usually need to provide legal proof of incorporation as a business.
Anyone can generate certificates, but some CAs are well-known organizations
with well-understood procedures for verifying the identity and good business
behavior of certificate applicants. For this reason, browsers ship with a list of
signing authorities that are trusted. If a browser receives a certificate signed by
some unknown (and possibly malicious) authority, the browser usually displays
a warning. Browsers also may choose to accept any certificates with a valid sign-
ing path to a trusted CA. In other words, if a trusted CA signs a certificate for
“Sam’s Signing Shop” and Sam’s Signing Shop signs a site certificate, the
browser may accept the certificate as deriving from a valid CA path.
Figure 14-17. HTTPS certificates are X.509 certificates with site information
Server
certificate
Client ServerInternet
Certificate serial number 35:DE:F4:CF
Certificate expiration date Wed, Sep 17, 2003
Site’s organization name Joe’s Hardware Online
Site’s DNS hostname www.joes-hardware.com
Site’s public key
Certificate issuer name RSA Data Security
Certificate issuer signature
328 |Chapter 14: Secure HTTP
Signature check
Once the signing authority is judged as trustworthy, the browser checks the cer-
tificate’s integrity by applying the signing authority’s public key to the signature
and comparing it to the checksum.
Site identity check
To prevent a server from copying someone else’s certificate or intercepting their
traffic, most browsers try to verify that the domain name in the certificate matches
the domain name of the server they talked to. Server certificates usually contain a
single domain name, but some CAs create certificates that contain lists of server
names or wildcarded domain names, for clusters or farms of servers. If the host-
name does not match the identity in the certificate, user-oriented clients must
either notify the user or terminate the connection with a bad certificate error.
Virtual Hosting and Certificates
It’s sometimes tricky to deal with secure traffic on sites that are virtually hosted (mul-
tiple hostnames on a single server). Some popular web server programs support only
a single certificate. If a user arrives for a virtual hostname that does not strictly match
the certificate name, a warning box is displayed.
For example, consider the Louisiana-themed e-commerce site Cajun-Shop.com. The
site’s hosting provider provided the official name cajun-shop.securesites.com. When
users go to https://www.cajun-shop.com, the official hostname listed in the server cer-
tificate (*.securesites.com) does not match the virtual hostname the user browsed to
(www.cajun-shop.com), and the warning in Figure 14-18 appears.
To prevent this problem, the owners of Cajun-Shop.com redirect all users to cajun-
shop.securesites.com when they begin secure transactions. Cert management for vir-
tually hosted sites can be a little tricky.
A Real HTTPS Client
SSL is a complicated binary protocol. Unless you are a crypto expert, you shouldn’t
send raw SSL traffic directly. Thankfully, several commercial and open source librar-
ies exist to make it easier to program SSL clients and servers.
OpenSSL
OpenSSL is the most popular open source implementation of SSL and TLS. The
OpenSSL Project is a collaborative volunteer effort to develop a robust, commercial-
grade, full-featured toolkit implementing the SSL and TLS protocols, as well as a full-
strength, general-purpose cryptography library. You can get information about
OpenSSL, and download the software, from http://www.openssl.org.
A Real HTTPS Client |329
You might also hear of SSLeay (pronounced S-S-L-e-a-y). OpenSSL is the successor
to the SSLeay library, and it has a very similar interface. SSLeay was originally devel-
oped by Eric A. Young (the “eay” of SSLeay).
A Simple HTTPS Client
In this section, we’ll use the OpenSSL package to write an extremely primitive
HTTPS client. This client establishes an SSL connection with a server, prints out
Figure 14-18. Certificate name mismatches bring up certificate error dialog boxes
(a) The hostname in this URL (www.cajun-shop.com)
does not match the name in the certificate, because the
site is virtually hosted, and the certificate is made out
to *.securesites.com.
(b) A dialog box warns the user that the site’s certificate has
a valid date and is from a valid certificate authority, but the
name listed in the certificate does not match the site
requested in the URL.
(c) To get more details the user presses the “View Certificate”
button, and sees that the certificate is a wildcard certificate
made out to “*.securesites.com”. With this information, the user
can decide whether to accept or decline the certificate.
(d) Accepting the certificate loads the page through the secure
HTTPS protocol.
To avoid this kind of user error, this particular site directs all
HTTPS traffic to the hostname alias cajun-shop.securesites.com.
This virtual hostname matches the name on the certificate
provided by the ISP as part of their commerce package.
330 |Chapter 14: Secure HTTP
some identification information from the site server, sends an HTTP GET request
across the secure channel, receives an HTTP response, and prints the response.
The C program shown below is an OpenSSL implementation of the trivial HTTPS
client. To keep the program simple, error-handling and certificate-processing logic
has not been included.
Because error handling has been removed from this example program, you should
use it only for explanatory value. The software will crash or otherwise misbehave in
normal error conditions.
/**********************************************************************
* https_client.c --- very simple HTTPS client with no error checking
* usage: https_client servername
**********************************************************************/
#include <stdio.h>
#include <memory.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <openssl/crypto.h>
#include <openssl/x509.h>
#include <openssl/pem.h>
#include <openssl/ssl.h>
#include <openssl/err.h>
void main(int argc, char **argv)
{
SSL *ssl;
SSL_CTX *ctx;
SSL_METHOD *client_method;
X509 *server_cert;
int sd,err;
char *str,*hostname,outbuf[4096],inbuf[4096],host_header[512];
struct hostent *host_entry;
struct sockaddr_in server_socket_address;
struct in_addr ip;
/*========================================*/
/* (1) initialize SSL library */
/*========================================*/
SSLeay_add_ssl_algorithms( );
client_method = SSLv2_client_method( );
SSL_load_error_strings( );
ctx = SSL_CTX_new(client_method);
A Real HTTPS Client |331
printf("(1) SSL context initialized\n\n");
/*=============================================*/
/* (2) convert server hostname into IP address */
/*=============================================*/
hostname = argv[1];
host_entry = gethostbyname(hostname);
bcopy(host_entry->h_addr, &(ip.s_addr), host_entry->h_length);
printf("(2) '%s' has IP address '%s'\n\n", hostname, inet_ntoa(ip));
/*=================================================*/
/* (3) open a TCP connection to port 443 on server */
/*=================================================*/
sd = socket (AF_INET, SOCK_STREAM, 0);
memset(&server_socket_address, '\0', sizeof(server_socket_address));
server_socket_address.sin_family = AF_INET;
server_socket_address.sin_port = htons(443);
memcpy(&(server_socket_address.sin_addr.s_addr),
host_entry->h_addr, host_entry->h_length);
err = connect(sd, (struct sockaddr*) &server_socket_address,
sizeof(server_socket_address));
if (err < 0) { perror("can't connect to server port"); exit(1); }
printf("(3) TCP connection open to host '%s', port %d\n\n",
hostname, server_socket_address.sin_port);
/*========================================================*/
/* (4) initiate the SSL handshake over the TCP connection */
/*========================================================*/
ssl = SSL_new(ctx); /* create SSL stack endpoint */
SSL_set_fd(ssl, sd); /* attach SSL stack to socket */
err = SSL_connect(ssl); /* initiate SSL handshake */
printf("(4) SSL endpoint created & handshake completed\n\n");
/*============================================*/
/* (5) print out the negotiated cipher chosen */
/*============================================*/
printf("(5) SSL connected with cipher: %s\n\n", SSL_get_cipher(ssl));
/*========================================*/
/* (6) print out the server's certificate */
/*========================================*/
server_cert = SSL_get_peer_certificate(ssl);
332 |Chapter 14: Secure HTTP
printf("(6) server's certificate was received:\n\n");
str = X509_NAME_oneline(X509_get_subject_name(server_cert), 0, 0);
printf(" subject: %s\n", str);
str = X509_NAME_oneline(X509_get_issuer_name(server_cert), 0, 0);
printf(" issuer: %s\n\n", str);
/* certificate verification would happen here */
X509_free(server_cert);
/*********************************************************/
/* (7) handshake complete --- send HTTP request over SSL */
/*********************************************************/
sprintf(host_header,"Host: %s:443\r\n",hostname);
strcpy(outbuf,"GET / HTTP/1.0\r\n");
strcat(outbuf,host_header);
strcat(outbuf,"Connection: close\r\n");
strcat(outbuf,"\r\n");
err = SSL_write(ssl, outbuf, strlen(outbuf));
shutdown (sd, 1); /* send EOF to server */
printf("(7) sent HTTP request over encrypted channel:\n\n%s\n",outbuf);
/**************************************************/
/* (8) read back HTTP response from the SSL stack */
/**************************************************/
err = SSL_read(ssl, inbuf, sizeof(inbuf) - 1);
inbuf[err] = '\0';
printf ("(8) got back %d bytes of HTTP response:\n\n%s\n",err,inbuf);
/************************************************/
/* (9) all done, so close connection & clean up */
/************************************************/
SSL_shutdown(ssl);
close (sd);
SSL_free (ssl);
SSL_CTX_free (ctx);
printf("(9) all done, cleaned up and closed connection\n\n");
}
This example compiles and runs on Sun Solaris, but it is illustrative of how SSL pro-
grams work on many OS platforms. This entire program, including all the encryp-
tion and key and certificate management, fits in a three-page C program, thanks to
the powerful features provided by OpenSSL.
A Real HTTPS Client |333
Let’s walk through the program section by section:
• The top of the program includes support files needed to support TCP network-
ing and SSL.
• Section 1 creates the local context that keeps track of the handshake parameters
and other state about the SSL connection, by calling SSL_CTX_new.
• Section 2 converts the input hostname (provided as a command-line argument)
to an IP address, using the Unix gethostbyname function. Other platforms may
have other ways to provide this facility.
• Section 3 opens a TCP connection to port 443 on the server by creating a local
socket, setting up the remote address information, and connecting to the remote
server.
• Once the TCP connection is established, we attach the SSL layer to the TCP con-
nection using SSL_new and SSL_set_fd and perform the SSL handshake with the
server by calling SSL_connect. When section 4 is done, we have a functioning
SSL channel established, with ciphers chosen and certificates exchanged.
• Section 5 prints out the value of the chosen bulk-encryption cipher.
• Section 6 prints out some of the information contained in the X.509 certificate
sent back from the server, including information about the certificate holder and
the organization that issued the certificate. The OpenSSL library doesn’t do any-
thing special with the information in the server certificate. A real SSL applica-
tion, such as a web browser, would do some sanity checks on the certificate to
make sure it is signed properly and came from the right host. We discussed what
browsers do with server certificates in “Site Certificate Validation.”
• At this point, our SSL connection is ready to use for secure data transfer. In sec-
tion 7, we send the simple HTTP request “GET / HTTP/1.0” over the SSL chan-
nel using SSL_write, then close the outbound half of the connection.
• In section 8, we read the response back from the connection using SSL_read, and
print it on the screen. Because the SSL layer takes care of all the encryption and
decryption, we can just write and read normal HTTP commands.
• Finally, we clean up in section 9.
Refer to http://www.openssl.org for more information about the OpenSSL libraries.
Executing Our Simple OpenSSL Client
The following shows the output of our simple HTTP client when pointed at a secure
server. In this case, we pointed the client at the home page of the Morgan Stanley
Online brokerage. Online trading companies make extensive use of HTTPS.
%https_client clients1.online.msdw.com
(1) SSL context initialized
334 |Chapter 14: Secure HTTP
(2) 'clients1.online.msdw.com' has IP address '63.151.15.11'
(3) TCP connection open to host 'clients1.online.msdw.com', port 443
(4) SSL endpoint created & handshake completed
(5) SSL connected with cipher: DES-CBC3-MD5
(6) server's certificate was received:
subject: /C=US/ST=Utah/L=Salt Lake City/O=Morgan Stanley/OU=Online/CN=
clients1.online.msdw.com
issuer: /C=US/O=RSA Data Security, Inc./OU=Secure Server Certification
Authority
(7) sent HTTP request over encrypted channel:
GET / HTTP/1.0
Host: clients1.online.msdw.com:443
Connection: close
(8) got back 615 bytes of HTTP response:
HTTP/1.1 302 Found
Date: Sat, 09 Mar 2002 09:43:42 GMT
Server: Stronghold/3.0 Apache/1.3.14 RedHat/3013c (Unix) mod_ssl/2.7.1 OpenSSL/0.9.6
Location: https://clients.online.msdw.com/cgi-bin/ICenter/home
Connection: close
Content-Type: text/html; charset=iso-8859-1
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<HTML><HEAD>
<TITLE>302 Found</TITLE>
</HEAD><BODY>
<H1>Found</H1>
The document has moved <A HREF="https://clients.online.msdw.com/cgi-bin/ICenter/
home">here</A>.<P>
<HR>
<ADDRESS>Stronghold/3.0 Apache/1.3.14 RedHat/3013c Server at clients1.online.msdw.com
Port 443</ADDRESS>
</BODY></HTML>
(9) all done, cleaned up and closed connection
As soon as the first four sections are completed, the client has an open SSL connec-
tion. It can then inquire about the state of the connection and chosen parameters
and can examine server certificates.
In this example, the client and server negotiated the DES-CBC3-MD5 bulk-encryption
cipher. You also can see that the server site certificate belongs to the organization
“Morgan Stanley” in “Salt Lake City, Utah, USA”. The certificate was granted by RSA
Data Security, and the hostname is “clients1.online.msdw.com,” which matches our
request.
Tunneling Secure Traffic Through Proxies |335
Once the SSL channel is established and the client feels comfortable about the site
certificate, it sends its HTTP request over the secure channel. In our example, the cli-
ent sends a simple “GET / HTTP/1.0” HTTP request and receives back a 302 Redi-
rect response, requesting that the user fetch a different URL.
Tunneling Secure Traffic Through Proxies
Clients often use web proxy servers to access web servers on their behalf (proxies are
discussed in Chapter 6). For example, many corporations place a proxy at the secu-
rity perimeter of the corporate network and the public Internet (Figure 14-19). The
proxy is the only device permitted by the firewall routers to exchange HTTP traffic,
and it may employ virus checking or other content controls.
But once the client starts encrypting the data to the server, using the server’s public
key, the proxy no longer has the ability to read the HTTP header! And if the proxy can-
not read the HTTP header, it won’t know where to forward the request (Figure 14-20).
To make HTTPS work with proxies, a few modifications are needed to tell the proxy
where to connect. One popular technique is the HTTPS SSL tunneling protocol.
Figure 14-19. Corporate firewall proxy
Figure 14-20. Proxy can’t proxy an encrypted request
Client
Client Firewall
proxy
Security
perimeter
Public Internet
bdfwr73ytr6ouydoiw687eqidfjwvd76weti76fig287hdi9
8r82yr87pfdy72y87193836PDUyqe719eyty3gee98y8787
client17.mycompany.com proxy.mycompany.com www.cajun-gifts.com
336 |Chapter 14: Secure HTTP
Using the HTTPS tunneling protocol, the client first tells the proxy the secure host
and port to which it wants to connect. It does this in plaintext, before encryption
starts, so the proxy can read this information.
HTTP is used to send the plaintext endpoint information, using a new extension
method called CONNECT. The CONNECT method tells the proxy to open a con-
nection to the desired host and port number and, when that’s done, to tunnel data
directly between the client and server. The CONNECT method is a one-line text
command that provides the hostname and port of the secure origin server, separated
by a colon. The host:port is followed by a space and an HTTP version string fol-
lowed by a CRLF. After that there is a series of zero or more HTTP request header
lines, followed by an empty line. After the empty line, if the handshake to establish
the connection was successful, SSL data transfer can begin. Here is an example:
CONNECT home.netscape.com:443 HTTP/1.0
User-agent: Mozilla/1.1N
<raw SSL-encrypted data would follow here...>
After the empty line in the request, the client will wait for a response from the proxy.
The proxy will evaluate the request and make sure that it is valid and that the user is
authorized to request such a connection. If everything is in order, the proxy will
make a connection to the destination server and, if successful, send a 200 Connec-
tion Established response to the client.
HTTP/1.0 200 Connection established
Proxy-agent: Netscape-Proxy/1.1
For more information about secure tunnels and security proxies, refer back to “Tun-
nels” in Chapter 8.
For More Information
Security and cryptography are hugely important and hugely complicated topics. If
you’d like to learn more about HTTP security, digital cryptography, digital certifi-
cates, and the Public-Key Infrastructure, here are a few starting points.
HTTP Security
Web Security, Privacy & Commerce
Simson Garfinkel, O’Reilly & Associates, Inc. This is one of the best, most read-
able introductions to web security and the use of SSL/TLS and digital certificates.
http://www.ietf.org/rfc/rfc2818.txt
RFC 2818, “HTTP Over TLS,” specifies how to implement secure HTTP over
Transport Layer Security (TLS), the modern successor to SSL.
For More Information |337
http://www.ietf.org/rfc/rfc2817.txt
RFC 2817, “Upgrading to TLS Within HTTP/1.1,” explains how to use the
Upgrade mechanism in HTTP/1.1 to initiate TLS over an existing TCP connec-
tion. This allows unsecured and secured HTTP traffic to share the same well-
known port (in this case, http: at 80 rather than https: at 443). It also enables
virtual hosting, so a single HTTP+TLS server can disambiguate traffic intended
for several hostnames at a single IP address.
SSL and TLS
http://www.ietf.org/rfc/rfc2246.txt
RFC 2246, “The TLS Protocol Version 1.0,” specifies Version 1.0 of the TLS pro-
tocol (the successor to SSL). TLS provides communications privacy over the
Internet. The protocol allows client/server applications to communicate in a way
that is designed to prevent eavesdropping, tampering, and message forgery.
http://developer.netscape.com/docs/manuals/security/sslin/contents.htm
“Introduction to SSL” introduces the Secure Sockets Layer (SSL) protocol. Origi-
nally developed by Netscape, SSL has been universally accepted on the World
Wide Web for authenticated and encrypted communication between clients and
servers.
http://www.netscape.com/eng/ssl3/draft302.txt
“The SSL Protocol Version 3.0” is Netscape’s 1996 specification for SSL.
http://developer.netscape.com/tech/security/ssl/howitworks.html
“How SSL Works” is Netscape’s introduction to key cryptography.
http://www.openssl.org
The OpenSSL Project is a collaborative effort to develop a robust, commercial-
grade, full-featured, and open source toolkit implementing the Secure Sockets
Layer (SSL v2/v3) and Transport Layer Security (TLS v1) protocols, as well as a
full-strength, general-purpose cryptography library. The project is managed by a
worldwide community of volunteers that use the Internet to communicate, plan,
and develop the OpenSSL toolkit and its related documentation. OpenSSL is
based on the excellent SSLeay library developed by Eric A. Young and Tim J.
Hudson. The OpenSSL toolkit is licensed under an Apache-style licence, which
basically means that you are free to get and use it for commercial and noncom-
mercial purposes, subject to some simple license conditions.
Public-Key Infrastructure
http://www.ietf.org/html.charters/pkix-charter.html
The IETF PKIX Working Group was established in 1995 with the intent of
developing Internet standards needed to support an X.509-based Public-Key
Infrastructure. This is a nice summary of that group’s activities.
338 |Chapter 14: Secure HTTP
http://www.ietf.org/rfc/rfc2459.txt
RFC 2459, “Internet X.509 Public Key Infrastructure Certificate and CRL Pro-
file,” contains details about X.509 v3 digital certificates.
Digital Cryptography
Applied Cryptography
Bruce Schneier, John Wiley & Sons. This is a classic book on cryptography for
implementors.
The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography
Simon Singh, Anchor Books. This entertaining book is a cryptography primer.
While it’s not intended for technology experts, it is a lively historical tour of
secret coding.
PART IV
Entities, Encodings, and
Internationalization
Part IV is all about the entity bodies of HTTP messages and the content that the
entity bodies ship around as cargo:
• Chapter 15, Entities and Encodings, describes the formats and syntax of HTTP
content.
• Chapter 16, Internationalization, surveys the web standards that allow people to
exchange content in different languages and different character sets, around the
globe.
• Chapter 17, Content Negotiation and Transcoding, explains mechanisms for
negotiating acceptable content.
341
CHAPTER 15
Entities and Encodings
HTTP ships billions of media objects of all kinds every day. Images, text, movies,
software programs... you name it, HTTP ships it. HTTP also makes sure that its
messages can be properly transported, identified, extracted, and processed. In partic-
ular, HTTP ensures that its cargo:
• Can be identified correctly (using Content-Type media formats and Content-
Language headers) so browsers and other clients can process the content properly
• Can be unpacked properly (using Content-Length and Content-Encoding headers)
• Is fresh (using entity validators and cache-expiration controls)
• Meets the user’s needs (based on content-negotiation Accept headers)
• Moves quickly and efficiently through the network (using range requests, delta
encoding, and other data compression)
• Arrives complete and untampered with (using transfer encoding headers and
Content-MD5 checksums)
To make all this happen, HTTP uses well-labeled entities to carry content.
This chapter discusses entities, their associated entity headers, and how they work to
transport web cargo. We’ll show how HTTP provides the essentials of content size,
type, and encodings. We’ll also explain some of the more complicated and powerful
features of HTTP entities, including range requests, delta encoding, digests, and
chunked encodings.
This chapter covers:
• The format and behavior of HTTP message entities as HTTP data containers
• How HTTP describes the size of entity bodies, and what HTTP requires in the
way of sizing
• The entity headers used to describe the format, alphabet, and language of con-
tent, so clients can process it properly
342 |Chapter 15: Entities and Encodings
• Reversible content encodings, used by senders to transform the content data for-
mat before sending to make it take up less space or be more secure
• Transfer encoding, which modifies how HTTP ships data to enhance the commu-
nication of some kinds of content, and chunked encoding, a transfer encoding
that chops data into multiple pieces to deliver content of unknown length safely
• The assortment of tags, labels, times, and checksums that help clients get the lat-
est version of requested content
• The validators that act like version numbers on content, so web applications can
ensure they have fresh content, and the HTTP header fields designed to control
object freshness
• Ranges, which are useful for continuing aborted downloads where they left off
• HTTP delta encoding extensions, which allow clients to request just those parts
of a web page that actually have changed since a previously viewed revision
• Checksums of entity bodies, which are used to detect changes in entity content
as it passes through proxies
Messages Are Crates, Entities Are Cargo
If you think of HTTP messages as the crates of the Internet shipping system, then
HTTP entities are the actual cargo of the messages. Figure 15-1 shows a simple
entity, carried inside an HTTP response message.
The entity headers indicate a plaintext document (Content-Type: text/plain) that is a
mere 18 characters long (Content-Length: 18). As always, a blank line (CRLF) sepa-
rates the header fields from the start of the body.
HTTP entity headers (covered in Chapter 3) describe the contents of an HTTP mes-
sage. HTTP/1.1 defines 10 primary entity header fields:
Content-Type
The kind of object carried by the entity.
Content-Length
The length or size of the message being sent.
Figure 15-1. Message entity is made up of entity headers and entity body
HTTP/1.0 200 OK
Server: Netscape-Enterprise/3.6
Date: Sun, 17 Sep 2000 00:01:05 GMT
Content-type: text/plain
Content-length: 18
Hi! I'm a message!
Entity headers
Entity body
Entity
Messages Are Crates, Entities Are Cargo |343
Content-Language
The human language that best matches the object being sent.
Content-Encoding
Any transformation (compression, etc.) performed on the object data.
Content-Location
An alternate location for the object at the time of the request.
Content-Range
If this is a partial entity, this header defines which pieces of the whole are included.
Content-MD5
A checksum of the contents of the entity body.
Last-Modified
The date on which this particular content was created or modified at the server.
Expires
The date and time at which this entity data will become stale.
Allow
What request methods are legal on this resource; e.g., GET and HEAD.
ETag
A unique validator for this particular instance*of the document. The ETag
header is not defined formally as an entity header, but it is an important header
for many operations involving entities.
Cache-Control
Directives on how this document can be cached. The Cache-Control header, like
the ETag header, is not defined formally as an entity header.
Entity Bodies
The entity body just contains the raw cargo.†Any other descriptive information is
contained in the headers. Because the entity body cargo is just raw data, the entity
headers are needed to describe the meaning of that data. For example, the Content-
Type entity header tells us how to interpret the data (image, text, etc.), and the Con-
tent-Encoding entity header tells us if the data was compressed or otherwise recoded.
We talk about all of this and more in upcoming sections.
The raw content begins immediately after the blank CRLF line that marks the end of
the header fields. Whatever the content is—text or binary, document or image, com-
pressed or uncompressed, English or French or Japanese—it is placed right after the
CRLF.
* Instances are described later in this chapter, in the section “Time-Varying Instances.”
† If there is a Content-Encoding header, the content already has been encoded by the content-encoding algo-
rithm, and the first byte of the entity is the first byte of the encoded (e.g., compressed) cargo.
344 |Chapter 15: Entities and Encodings
Figure 15-2 shows two examples of real HTTP messages, one carrying a text entity,
the other carrying an image entity. The hexadecimal values show the exact contents
of the message:
• In Figure 15-2a, the entity body begins at byte number 65, right after the end-of-
headers CRLF. The entity body contains the ASCII characters for “Hi! I’m a
message!”
• In Figure 15-2b, the entity body begins at byte number 67. The entity body con-
tains the binary contents of the GIF image. GIF files begin with 6-byte version
signature, a 16-bit width, and a 16-bit height. You can see all three of these
directly in the entity body.
Content-Length: The Entity’s Size
The Content-Length header indicates the size of the entity body in the message, in
bytes. The size includes any content encodings (the Content-Length of a gzip-
compressed text file will be the compressed size, not the original size).
The Content-Length header is mandatory for messages with entity bodies, unless the
message is transported using chunked encoding. Content-Length is needed to detect
premature message truncation when servers crash and to properly segment messages
that share a persistent connection.
Detecting Truncation
Older versions of HTTP used connection close to delimit the end of a message. But,
without Content-Length, clients cannot distinguish between successful connection
Figure 15-2. Hex dumps of real message content (raw message content follows blank CRLF)
final LF
(0x0A= <LF>) start-of-content
(”GIF87a”)Width
(0x0227= 551) Height
(0x0206= 518)
HTTP/1.0 200 OK
Content-type: text/plain
Content-length: 18
Hi! I’m a message!
final LF (0x0A= <LF>) start-of-content (0x48= “H”)
(b) Image/gif entity in HTTP response message
HTTP/1.0 200 OK
Content-Type: image/gif
Content-Length: 34867
(a) Text/plain entity in HTTP response message
Content-Length: The Entity’s Size |345
close at the end of a message and connection close due to a server crash in the mid-
dle of a message. Clients need Content-Length to detect message truncation.
Message truncation is especially severe for caching proxy servers. If a cache receives a
truncated message and doesn’t recognize the truncation, it may store the defective
content and serve it many times. Caching proxy servers generally do not cache HTTP
bodies that don’t have an explicit Content-Length header, to reduce the risk of cach-
ing truncated messages.
Incorrect Content-Length
An incorrect Content-Length can cause even more damage than a missing Content-
Length. Because some early clients and servers had well-known bugs with respect to
Content-Length calculations, some clients, servers, and proxies contain algorithms to
try to detect and correct interactions with broken servers. HTTP/1.1 user agents offi-
cially are supposed to notify the user when an invalid length is received and detected.
Content-Length and Persistent Connections
Content-Length is essential for persistent connections. If the response comes across a
persistent connection, another HTTP response can immediately follow the current
response. The Content-Length header lets the client know where one message ends
and the next begins. Because the connection is persistent, the client cannot use con-
nection close to identify the message’s end. Without a Content-Length header, HTTP
applications won’t know where one entity body ends and the next message begins.
As we will see in “Transfer Encoding and Chunked Encoding,” there is one situation
where you can use persistent connections without having a Content-Length header:
when you use chunked encoding. Chunked encoding sends the data in a series of
chunks, each with a specified size. Even if the server does not know the size of the
entire entity at the time the headers are generated (often because the entity is being
generated dynamically), the server can use chunked encoding to transmit pieces of
well-defined size.
Content Encoding
HTTP lets you encode the contents of an entity body, perhaps to make it more
secure or to compress it to take up less space (we explain compression in detail later
in this chapter). If the body has been content-encoded, the Content-Length header
specifies the length, in bytes, of the encoded body, not the length of the original,
unencoded body.
Some HTTP applications have been known to get this wrong and to send the size of
the data before the encoding, which causes serious errors, especially with persis-
tent connections. Unfortunately, none of the headers described in the HTTP/1.1
346 |Chapter 15: Entities and Encodings
specification can be used to send the length of the original, unencoded body, which
makes it difficult for clients to verify the integrity of their unencoding processes.*
Rules for Determining Entity Body Length
The following rules describe how to correctly determine the length and end of an
entity body in several different circumstances. The rules should be applied in order;
the first match applies.
1. If a particular HTTP message type is not allowed to have a body, ignore the
Content-Length header for body calculations. The Content-Length headers are
informational in this case and do not describe the actual body length. (Naïve
HTTP applications can get in trouble if they assume Content-Length always
means there is a body).
The most important example is the HEAD response. The HEAD method
requests that a server send the headers that would have been returned by an
equivalent GET request, but no body. Because a GET response would send back
a Content-Length header, so will the HEAD response—but unlike the GET
response, the HEAD response will not have a body. 1XX, 204, and 304
responses also can have informational Content-Length headers but no entity
body. Messages that forbid entity bodies must terminate at the first empty line
after the headers, regardless of which entity header fields are present.
2. If a message contains a Transfer-Encoding header (other than the default HTTP
“identity” encoding), the entity will be terminated by a special pattern called a
“zero-byte chunk,” unless the message is terminated first by closing the connec-
tion. We’ll discuss transfer encodings and chunked encodings later in this chapter.
3. If a message has a Content-Length header (and the message type allows entity
bodies), the Content-Length value contains the body length, unless there is a
non-identity Transfer-Encoding header. If a message is received with both a
Content-Length header field and a non-identity Transfer-Encoding header field,
you must ignore the Content-Length, because the transfer encoding will change
the way entity bodies are represented and transferred (and probably the number
of bytes transmitted).
4. If the message uses the “multipart/byteranges” media type and the entity length
is not otherwise specified (in the Content-Length header), each part of the multi-
part message will specify its own size. This multipart type is the only entity body
type that self-delimits its own size, so this media type must not be sent unless the
sender knows the recipient can parse it.†
* Even the Content-MD5 header, which can be used to send the 128-bit MD5 of the document, contains the
MD5 of the encoded document. The Content-MD5 header is described later in this chapter.
† Because a Range header might be forwarded by a more primitive proxy that does not understand multipart/
byteranges, the sender must delimit the message using methods 1, 3, or 5 in this section if it isn’t sure the
receiver understands the self- delimiting format.
Entity Digests |347
5. If none of the above rules match, the entity ends when the connection closes.
In practice, only servers can use connection close to indicate the end of a
message. Clients can’t close the connection to signal the end of client mes-
sages, because that would leave no way for the server to send back a
response.*
6. To be compatible with HTTP/1.0 applications, any HTTP/1.1 request that has
an entity body also must include a valid Content-Length header field (unless the
server is known to be HTTP/1.1-compliant). The HTTP/1.1 specification coun-
sels that if a request contains a body and no Content-Length, the server should
send a 400 Bad Request response if it cannot determine the length of the mes-
sage, or a 411 Length Required response if it wants to insist on receiving a valid
Content-Length.
Entity Digests
Although HTTP typically is implemented over a reliable transport protocol such
as TCP/IP, parts of messages may get modified in transit for a variety of reasons,
such as noncompliant transcoding proxies or buggy intermediary proxies. To
detect unintended (or undesired) modification of entity body data, the sender
can generate a checksum of the data when the initial entity is generated, and the
receiver can sanity check the checksum to catch any unintended entity modifica-
tion.†
The Content-MD5 header is used by servers to send the result of running the
MD5 algorithm on the entity body. Only the server where the response origi-
nates may compute and send the Content-MD5 header. Intermediate proxies and
caches may not modify or add the header—that would violate the whole pur-
pose of verifying end-to-end integrity. The Content-MD5 header contains the
MD5 of the content after all content encodings have been applied to the entity
body and before any transfer encodings have been applied to it. Clients seeking
to verify the integrity of the message must first decode the transfer encodings,
then compute the MD5 of the resulting unencoded entity body. As an example, if
a document is compressed using the gzip algorithm, then sent with chunked
encoding, the MD5 algorithm is run on the full gripped body.
In addition to checking message integrity, the MD5 can be used as a key into a
hash table to quickly locate documents and reduce duplicate storage of content.
Despite these possible uses, the Content-MD5 header is not sent often.
* The client could do a half close of just its output connection, but many server applications aren’t designed
to handle this situation and will interpret a half close as the client disconnecting from the server. Connection
management was never well specified in HTTP. See Chapter 4 for more details.
† This method, of course, is not immune to a malicious attack that replaces both the message body and digest
header. It is intended only to detect unintentional modification. Other facilities, such as digest authentica-
tion, are needed to provide safeguards against malicious tampering.
348 |Chapter 15: Entities and Encodings
Extensions to HTTP have proposed other digest algorithms in IETF drafts. These
extensions have proposed a new header, Want-Digest, that allows clients to specify
the type of digest they expect with the response. Quality values can be used to sug-
gest multiple digest algorithms and indicate preference.
Media Type and Charset
The Content-Type header field describes the MIME type of the entity body.*The
MIME type is a standardized name that describes the underlying type of media car-
ried as cargo (HTML file, Microsoft Word document, MPEG video, etc.). Client
applications use the MIME type to properly decipher and process the content.
The Content-Type values are standardized MIME types, registered with the Internet
Assigned Numbers Authority (IANA). MIME types consist of a primary media type
(e.g., text, image, audio), followed by a slash, followed by a subtype that further
specifies the media type. Table 15-1 lists a few common MIME types for the Content-
Type header. More MIME types are listed in Appendix D.
It is important to note that the Content-Type header specifies the media type of the
original entity body. If the entity has gone through content encoding, for example,
the Content-Type header will still specify the entity body type before the encoding.
* In the case of the HEAD request, Content-Type shows the type that would have been sent if it was a GET
request.
Table 15-1. Common media types
Media type Description
text/html Entity body is an HTML document
text/plain Entity body is a document in plain text
image/gif Entity body is an image of type GIF
image/jpeg Entity body is an image of type JPEG
audio/x-wav Entity body contains WAV sound data
model/vrml Entity body is a three-dimensional VRML model
application/vnd.ms-powerpoint Entity body is a Microsoft PowerPoint presentation
multipart/byteranges Entity body has multiple parts, each containing a different range (in bytes) of the full doc-
ument
message/http Entity body contains a complete HTTP message (see TRACE)
Media Type and Charset |349
Character Encodings for Text Media
The Content-Type header also supports optional parameters to further specify the
content type. The “charset” parameter is the primary example, specifying the mecha-
nism to convert bits from the entity into characters in a text file:
Content-Type: text/html; charset=iso-8859-4
We talk about character sets in detail in Chapter 16.
Multipart Media Types
MIME “multipart” email messages contain multiple messages stuck together and
sent as a single, complex message. Each component is self-contained, with its own
set of headers describing its content; the different components are concatenated
together and delimited by a string.
HTTP also supports multipart bodies; however, they typically are sent in only one of
two situations: in fill-in form submissions and in range responses carrying pieces of a
document.
Multipart Form Submissions
When an HTTP fill-in form is submitted, variable-length text fields and uploaded
objects are sent as separate parts of a multipart body, allowing forms to be filled out
with values of different types and lengths. For example, you may choose to fill out a
form that asks for your name and a description with your nickname and a small
photo, while your friend may put down her full name and a long essay describing her
passion for fixing Volkswagen buses.
HTTP sends such requests with a Content-Type: multipart/form-data header or a
Content-Type: multipart/mixed header and a multipart body, like this:
Content-Type: multipart/form-data; boundary=[abcdefghijklmnopqrstuvwxyz]
where the boundary specifies the delimiter string between the different parts of the
body.
The following example illustrates multipart/form-data encoding. Suppose we have
this form:
<FORM action="http://server.com/cgi/handle"
enctype="multipart/form-data"
method="post">
<P>
What is your name? <INPUT type="text" name="submit-name"><BR>
What files are you sending? <INPUT type="file" name="files"><BR>
<INPUT type="submit" value="Send"> <INPUT type="reset">
</FORM>
350 |Chapter 15: Entities and Encodings
If the user enters “Sally” in the text-input field and selects the text file “essayfile.txt,”
the user agent might send back the following data:
Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="submit-name"
Sally
--AaB03x
Content-Disposition: form-data; name="files"; filename="essayfile.txt"
Content-Type: text/plain
...contents of essayfile.txt...
--AaB03x--
If the user selected a second (image) file, “imagefile.gif,” the user agent might con-
struct the parts as follows:
Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="submit-name"
Sally
--AaB03x
Content-Disposition: form-data; name="files"
Content-Type: multipart/mixed; boundary=BbC04y
--BbC04y
Content-Disposition: file; filename="essayfile.txt"
Content-Type: text/plain
...contents of essayfile.txt...
--BbC04y
Content-Disposition: file; filename="imagefile.gif"
Content-Type: image/gif
Content-Transfer-Encoding: binary
...contents of imagefile.gif...
--BbC04y--
--AaB03x--
Multipart Range Responses
HTTP responses to range requests also can be multipart. Such responses come with a
Content-Type: multipart/byteranges header and a multipart body with the different
ranges. Here is an example of a multipart response to a request for different ranges of
a document:
HTTP/1.0 206 Partial content
Server: Microsoft-IIS/5.0
Date: Sun, 10 Dec 2000 19:11:20 GMT
Content-Location: http://www.joes-hardware.com/gettysburg.txt
Content-Type: multipart/x-byteranges; boundary=--[abcdefghijklmnopqrstuvwxyz]--
Last-Modified: Sat, 09 Dec 2000 00:38:47 GMT
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 0-174/1441
Content Encoding |351
Fourscore and seven years ago our fathers brough forth on this continent
a new nation, conceived in liberty and dedicated to the proposition that
all men are created equal.
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 552-761/1441
But in a larger sense, we can not dedicate, we can not consecrate,
we can not hallow this ground. The brave men, living and dead who
struggled here have consecrated it far above our poor power to add
or detract.
--[abcdefghijklmnopqrstuvwxyz]--
Content-Type: text/plain
Content-Range: bytes 1344-1441/1441
and that government of the people, by the people, for the people shall
not perish from the earth.
--[abcdefghijklmnopqrstuvwxyz]--
Range requests are discussed in more detail later in this chapter.
Content Encoding
HTTP applications sometimes want to encode content before sending it. For exam-
ple, a server might compress a large HTML document before sending it to a client
that is connected over a slow connection, to help lessen the time it takes to transmit
the entity. A server might scramble or encrypt the contents in a way that prevents
unauthorized third parties from viewing the contents of the document.
These types of encodings are applied to the content at the sender. Once the content
is content-encoded, the encoded data is sent to the receiver in the entity body as
usual.
The Content-Encoding Process
The content-encoding process is:
1. A web server generates an original response message, with original Content-
Type and Content-Length headers.
2. A content-encoding server (perhaps the origin server or a downstream proxy)
creates an encoded message. The encoded message has the same Content-Type
but (if, for example, the body is compressed) a different Content-Length. The
content-encoding server adds a Content-Encoding header to the encoded mes-
sage, so that a receiving application can decode it.
3. A receiving program gets the encoded message, decodes it, and obtains the
original.
352 |Chapter 15: Entities and Encodings
Figure 15-3 sketches a content-encoding example.
Here, an HTML page is encoded by a gzip content-encoding function, to produce a
smaller, compressed body. The compressed body is sent across the network, flagged
with the gzip encoding. The receiving client decompresses the entity using the gzip
decoder.
This response snippet shows another example of an encoded response (a com-
pressed image):
HTTP/1.1 200 OK
Date: Fri, 05 Nov 1999 22:35:15 GMT
Server: Apache/1.2.4
Content-Length: 6096
Content-Type: image/gif
Content-Encoding: gzip
[...]
Note that the Content-Type header can and should still be present in the message. It
describes the original format of the entity—information that may be necessary for
displaying the entity once it has been decoded. Remember that the Content-Length
header now represents the length of the encoded body.
Content-Encoding Types
HTTP defines a few standard content-encoding types and allows for additional
encodings to be added as extension encodings. Encodings are standardized through
the IANA, which assigns a unique token to each content-encoding algorithm. The
Content-Encoding header uses these standardized token values to describe the algo-
rithm used in the encoding.
Some of the common content-encoding tokens are listed in Table 15-2.
Figure 15-3. Content-encoding example
Gzip content
decoder
Gzip content
encoder
Content-type: text/html
Content-length: 12480
Original content
Content-type: text/html
Content-length: 3907
Content-encoding: gzip
Content-encoded content
Content-type: text/html
Content-length: 12480
Original content
01001011
11000101
Content Encoding |353
The gzip, compress, and deflate encodings are lossless compression algorithms used
to reduce the size of transmitted messages without loss of information. Of these, gzip
typically is the most effective compression algorithm and is the most widely used.
Accept-Encoding Headers
Of course, we don’t want servers encoding content in ways that the client can’t deci-
pher. To prevent servers from using encodings that the client doesn’t support, the
client passes along a list of supported content encodings in the Accept-Encoding
request header. If the HTTP request does not contain an Accept-Encoding header, a
server can assume that the client will accept any encoding (equivalent to passing
Accept-Encoding: *).
Figure 15-4 shows an example of Accept-Encoding in an HTTP transaction.
Table 15-2. Content-encoding tokens
Content-encoding value Description
gzip Indicates that the GNU zip encoding was applied to the entity.a
aRFC 1952 describes the gzip encoding.
compress Indicates that the Unix file compression program has been run on the entity.
deflate Indicates that the entity has been compressed into the zlib format.b
bRFCs 1950 and 1951 describe the zlib format and deflate compression.
identity Indicates that no encoding has been performed on the entity. When a Content-Encoding header
is not present, this can be assumed.
Figure 15-4. Content encoding
Request message
GET /logo.gif HTTP/1.1
Accept-encoding: gzip
[...]
HTTP/1.1 200 OK
Content-type: image/gif
Content-encoding: gzip
[...]
Response message
gzip
...011010011...
gunzip
...011010011...
The server compresses the image with gzip to transport a smaller file over the thin
network connection between itself and the client. This saves network bandwidth
and reduces the amount of time that the client waits for the transfer. Though, the
client will have to spend time decompressing the image once the image is served.
354 |Chapter 15: Entities and Encodings
The Accept-Encoding field contains a comma-separated list of supported encodings.
Here are a few examples:
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
Clients can indicate preferred encodings by attaching Q (quality) values as parame-
ters to each encoding. Q values can range from 0.0, indicating that the client does
not want the associated encoding, to 1.0, indicating the preferred encoding. The
token “*” means “anything else.” The process of selecting which content encoding to
apply is part of a more general process of deciding which content to send back to a
client in a response. This process and the Content-Encoding and Accept-Encoding
headers are discussed in more detail in Chapter 17.
The identity encoding token can be present only in the Accept-Encoding header and is
used by clients to specify relative preference over other content-encoding algorithms.
Transfer Encoding and Chunked Encoding
The previous section discussed content encodings—reversible transformations applied
to the body of the message. Content encodings are tightly associated with the details
of the particular content format. For example, you might compress a text file with
gzip, but not a JPEG file, because JPEGs don’t compress well with gzip.
This section discusses transfer encodings. Transfer encodings also are reversible
transformations performed on the entity body, but they are applied for architectural
reasons and are independent of the format of the content. You apply a transfer
encoding to a message to change the way message data is transferred across the net-
work (Figure 15-5).
Safe Transport
Historically, transfer encodings exist in other protocols to provide “safe transport” of
messages across a network. The concept of safe transport has a different focus for
HTTP, where the transport infrastructure is standardized and more forgiving. In
HTTP, there are only a few reasons why transporting message bodies can cause trou-
ble. Two of these are:
Unknown size
Some gateway applications and content encoders are unable to determine the
final size of a message body without generating the content first. Often, these
servers would like to start sending the data before the size is known. Because
Transfer Encoding and Chunked Encoding |355
HTTP requires the Content-Length header to precede the data, some servers
apply a transfer encoding to send the data with a special terminating footer that
indicates the end of data.*
Security
You might use a transfer encoding to scramble the message content before send-
ing it across a shared transport network. However, because of the popularity of
transport layer security schemes like SSL, transfer-encoding security isn’t very
common.
Transfer-Encoding Headers
There are just two defined headers to describe and control transfer encoding:
Transfer-Encoding
Tells the receiver what encoding has been performed on the message in order for
it to be safely transported
TE
Used in the request header to tell the server what extension transfer encodings
are okay to use†
Figure 15-5. Content encodings versus transfer encodings
* You could close the connection as a “poor man’s” end-of-message signal, but this breaks persistent
connections.
† The meaning of the TE header would be more intuitive if it were called the Accept-Transfer-Encoding header.
Normal header block
Normal entity
(just encoded)
HTTP/1.0 200 OK
Content-encoding: gzip
Content-type: text/html
[...]
[encoded message]
Content-encoded response
Basic header
HTTP/1.1 200 OK
Transfer-encoding: chunked
10
abcdefghijk
1
a
Transfer-encoded response
Encoded blocks
A Content-encoded message just encodes the entity
section of the message. With Transfer-encoded
messages the encoding is a function of the entire
message, changing the structure of the message itself.
356 |Chapter 15: Entities and Encodings
In the following example, the request uses the TE header to tell the server that it
accepts the chunked encoding (which it must if it’s an HTTP 1.1 application) and is
willing to accept trailers on the end of chunk-encoded messages:
GET /new_products.html HTTP/1.1
Host: www.joes-hardware.com
User-Agent: Mozilla/4.61 [en] (WinNT; I)
TE: trailers, chunked
...
The response includes a Transfer-Encoding header to tell the receiver that the mes-
sage has been transfer-encoded with the chunked encoding:
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Server: Apache/3.0
...
After this initial header, the structure of the message will change.
All transfer-encoding values are case-insensitive. HTTP/1.1 uses transfer-encoding
values in the TE header field and in the Transfer-Encoding header field. The latest
HTTP specification defines only one transfer encoding, chunked encoding.
The TE header, like the Accept-Encoding header, can have Q values to describe pre-
ferred forms of transfer encoding. The HTTP/1.1 specification, however, forbids the
association of a Q value of 0.0 to chunked encoding.
Future extensions to HTTP may drive the need for additional transfer encodings. If
and when this happens, the chunked transfer encoding should always be applied on
top of the extension transfer encodings. This guarantees that the data will get “tun-
neled” through HTTP/1.1 applications that understand chunked encoding but not
other transfer encodings.
Chunked Encoding
Chunked encoding breaks messages into chunks of known size. Each chunk is sent
one after another, eliminating the need for the size of the full message to be known
before it is sent.
Note that chunked encoding is a form of transfer encoding and therefore is an
attribute of the message, not the body. Multipart encoding, described earlier in this
chapter, is an attribute of the body and is completely separate from chunked encoding.
Chunking and persistent connections
When the connection between the client and server is not persistent, clients do not
need to know the size of the body they are reading—they expect to read the body
until the server closes the connection.
Transfer Encoding and Chunked Encoding |357
With persistent connections, the size of the body must be known and sent in the
Content-Length header before the body can be written. When content is dynami-
cally created at a server, it may not be possible to know the length of the body before
sending it.
Chunked encoding provides a solution for this dilemma, by allowing servers to send
the body in chunks, specifying only the size of each chunk. As the body is dynami-
cally generated, a server can buffer up a portion of it, send its size and the chunk,
and then repeat the process until the full body has been sent. The server can signal
the end of the body with a chunk of size 0 and still keep the connection open and
ready for the next response.
Chunked encoding is fairly simple. Figure 15-6 shows the basic anatomy of a chunked
message. It begins with an initial HTTP response header block, followed by a stream
of chunks. Each chunk contains a length value and the data for that chunk. The length
value is in hexadecimal form and is separated from the chunk data with a CRLF. The
size of the chunk data is measured in bytes and includes neither the CRLF sequence
between the length value and the data nor the CRLF sequence at the end of the chunk.
The last chunk is special—it has a length of zero, which signifies “end of body.”
Figure 15-6. Anatomy of a chunked message
HTTP/1.1 200 OK<CR><LF>
Content-type: text/plain<CR><LF>
Transfer-encoding: chunked<CR><LF>
Trailer: Content-MD5<CR><LF>
<CR><LF>
27<CR><LF>
We hold these truths to be self-evident<CR><LF>
26<CR><LF>
, that all men are created equal, that<CR><LF>
84<CR><LF>
they are endowed by their Creator with certain
unalienable Rights, that among these are Life,
Liberty and the pursuit of Happiness.<CR><LF>
0<CR><LF>
Content-MD5:gjqei54p26tjisgj3p4utjgrj53<CR><LF>
HTTP response
Chunk #1
Chunk #2
Chunk #3
Last chunk
Trailer*
Response
stream
Hexadecimal chunk size (27 hex=> 39 characters)
*Optional–only present if there is a Trailer header in the message headers.
358 |Chapter 15: Entities and Encodings
A client also may send chunked data to a server. Because the client does not know
beforehand whether the server accepts chunked encoding (servers do not send TE
headers in responses to clients), it must be prepared for the server to reject the
chunked request with a 411 Length Required response.
Trailers in chunked messages
A trailer can be added to a chunked message if the client’s TE header indicates that it
accepts trailers, or if the trailer is added by the server that created the original
response and the contents of the trailer are optional metadata that it is not necessary
for the client to understand and use (it is okay for the client to ignore and discard the
contents of the trailer).*
The trailer can contain additional header fields whose values might not have been
known at the start of the message (e.g., because the contents of the body had to be
generated first). An example of a header that can be sent in the trailer is the Content-
MD5 header—it would be difficult to calculate the MD5 of a document before the
document has been generated. Figure 15-6 illustrates the use of trailers. The message
headers contain a Trailer header listing the headers that will follow the chunked mes-
sage. The last chunk is followed by the headers listed in the Trailer header.
Any of the HTTP headers can be sent as trailers, except for the Transfer-Encoding,
Trailer, and Content-Length headers.
Combining Content and Transfer Encodings
Content encoding and transfer encoding can be used simultaneously. For example,
Figure 15-7 illustrates how a sender can compress an HTML file using a content
encoding and send the data chunked using a transfer encoding. The process to
“reconstruct” the body is reversed on the receiver.
Transfer-Encoding Rules
When a transfer encoding is applied to a message body, a few rules must be followed:
• The set of transfer encodings must include “chunked.” The only exception is if
the message is terminated by closing the connection.
• When the chunked transfer encoding is used, it is required to be the last transfer
encoding applied to the message body.
• The chunked transfer encoding must not be applied to a message body more
than once.
* The Trailer header was added after the initial chunked encoding was added to drafts of the HTTP/1.1 spec-
ification, so some applications may not understand it (or understand trailers) even if they claim to be
HTTP/1.1-compliant.
Time-Varying Instances |359
These rules allow the recipient to determine the transfer length of the message.
Transfer encodings are a relatively new feature of HTTP, introduced in Version 1.1.
Servers that implement transfer encodings need to take special care not to send
transfer-encoded messages to non-HTTP/1.1 applications. Likewise, if a server
receives a transfer-encoded message that it can not understand, it should respond
with the 501 Unimplemented status code. However, all HTTP/1.1 applications must
at least support chunked encoding.
Time-Varying Instances
Web objects are not static. The same URL can, over time, point to different versions of
an object. Take the CNN home page as an example—going to “http://www.cnn.com”
several times in a day is likely to result in a slightly different page being returned each
time.
Think of the CNN home page as being an object and its different versions as being
different instances of the object (see Figure 15-8). The client in the figure requests the
same resource (URL) multiple times, but it gets different instances of the resource as
it changes over time. At time (a) and (b) it has the same instance; at time (c) it has a
different instance.
The HTTP protocol specifies operations for a class of requests and responses, called
instance manipulations, that operate on instances of an object. The two main
instance-manipulation methods are range requests and delta encoding. Both of these
methods require clients to be able to identify the exact copy of the resource that they
have (if any) and request new instances conditionally. These mechanisms are dis-
cussed later in this chapter.
Figure 15-7. Combining content encoding with transfer encoding
Content-type: text/html
Content encoding
9BF2578EA4
2670CD
Content-type: text/html
Content-encoding: gzip
Transfer encoding
(chunking)
Content-type: text/html
Content-encoding: gzip
Transfer-encoding: chunked
426
8EA
257
9BF
9BF2578EA4
2670CD
9BF
257
8EA
426
360 |Chapter 15: Entities and Encodings
Validators and Freshness
Look back at Figure 15-8. The client does not initially have a copy of the resource, so
it sends a request to the server asking for it. The server responds with Version 1 of
the resource. The client can now cache this copy, but for how long?
Once the document has “expired” at the client (i.e., once the client can no longer
consider its copy a valid copy), it must request a fresh copy from the server. If the
document has not changed at the server, however, the client does not need to receive
it again—it can just continue to use its cached copy.
This special request, called a conditional request, requires that the client tell the server
which version it currently has, using a validator, and ask for a copy to be sent only if
its current copy is no longer valid. Let’s look at the three key concepts—freshness,
validators, and conditionals—in more detail.
Freshness
Servers are expected to give clients information about how long clients can cache
their content and consider it fresh. Servers can provide this information using one of
two headers: Expires and Cache-Control.
The Expires header specifies the exact date and time when the document
“expires”—when it can no longer be considered fresh. The syntax for the Expires
header is:
Expires: Sun Mar 18 23:59:59 GMT 2001
For a client and server to use the Expires header correctly, their clocks must be syn-
chronized. This is not always easy, because neither may run a clock synchronization
protocol such as the Network Time Protocol (NTP). A mechanism that defines expi-
ration using relative time is more useful. The Cache-Control header can be used to
specify the maximum age for a document in seconds—the total amount of time since
the document left the server. Age is not dependent on clock synchronization and
therefore is likely to yield more accurate results.
Figure 15-8. Instances are “snapshots” of a resource in time
V1 V1 V2 V2 V4
Time
(a)
Feb 17
4:30 p.m.
Version 1
(b)
Mar 3
11:21 a.m.
Version 2
(c) (d)
Apr 2
9:07 a.m.
Version 3
(e)
Apr 12
1:48 p.m.
Version 4 www.cnn.com
Validators and Freshness |361
The Cache-Control header actually is very powerful. It can be used by both servers
and clients to describe freshness using more directives than just specifying an age or
expiration time. Table 15-3 lists some of the directives that can accompany the
Cache-Control header.
Caching and freshness were discussed in more detail in Chapter 7.
Conditionals and Validators
When a cache’s copy is requested, and it is no longer fresh, the cache needs to make
sure it has a fresh copy. The cache can fetch the current copy from the origin server,
but in many cases, the document on the server is still the same as the stale copy in
the cache. We saw this in Figure 15-8b; the cached copy may have expired, but the
Table 15-3. Cache-Control header directives
Directive Message type Description
no-cache Request Do not return a cached copy of the document without first revalidating it with the
server.
no-store Request Do not return a cached copy of the document. Do not store the response from the
server.
max-age Request The document in the cache must not be older than the specified age.
max-stale Request The document may be stale based on the server-specified expiration information,
but it must not have been expired for longer than the value in this directive.
min-fresh Request The document’s age must not be more than its age plus the specified amount. In
other words, the response must be fresh for at least the specified amount of time.
no-transform Request The document must not be transformed before being sent.
only-if-cached Request Send the document only if it is in the cache, without contacting the origin server.
public Response Response may be cached by any cache.
private Response Response may be cached such that it can be accessed only by a single client.
no-cache Response If the directive is accompanied by a list of header fields, the content may be
cached and served to clients, but the listed header fields must first be removed. If
no header fields are specified, the cached copy must not be served without revali-
dation with the server.
no-store Response Response must not be cached.
no-transform Response Response must not be modified in any way before being served.
must-revalidate Response Response must be revalidated with the server before being served.
proxy-revalidate Response Shared caches must revalidate the response with the origin server before serving.
This directive can be ignored by private caches.
max-age Response Specifies the maximum length of time the document can be cached and still con-
sidered fresh.
s-max-age Response Specifies the maximum age of the document as it applies to shared caches (over-
riding the max-age directive, if one is present). This directive can be ignored by
private caches.
362 |Chapter 15: Entities and Encodings
server content still is the same as the cache content. If a cache always fetches a
server’s document, even if it’s the same as the expired cache copy, the cache wastes
network bandwidth, places unnecessary load on the cache and server, and slows
everything down.
To fix this, HTTP provides a way for clients to request a copy only if the resource has
changed, using special requests called conditional requests. Conditional requests are
normal HTTP request messages, but they are performed only if a particular condi-
tion is true. For example, a cache might send the following conditional GET message
to a server, asking it to send the file /announce.html only if the file has been modified
since June 29, 2002 (the date the cached document was last changed by the author):
GET /announce.html HTTP/1.0
If-Modified-Since: Sat, 29 Jun 2002, 14:30:00 GMT
Conditional requests are implemented by conditional headers that start with “If-”. In
the example above, the conditional header is If-Modified-Since. A conditional header
allows a method to execute only if the condition is true. If the condition is not true,
the server sends an HTTP error code back.
Each conditional works on a particular validator. A validator is a particular attribute
of the document instance that is tested. Conceptually, you can think of the validator
like the serial number, version number, or last change date of a document. A wise cli-
ent in Figure 15-8b would send a conditional validation request to the server saying,
“send me the resource only if it is no longer Version 1; I have Version 1.” We dis-
cussed conditional cache revalidation in Chapter 7, but we’ll study the details of
entity validators more carefully in this chapter.
The If-Modified-Since conditional header tests the last-modified date of a document
instance, so we say that the last-modified date is the validator. The If-None-Match
conditional header tests the ETag value of a document, which is a special keyword or
version-identifying tag associated with the entity. Last-Modified and ETag are the
two primary validators used by HTTP. Table 15-4 lists four of the HTTP headers
used for conditional requests. Next to each conditional header is the type of valida-
tor used with the header.
Table 15-4. Conditional request types
Request type Validator Description
If-Modified-Since Last-Modified Send a copy of the resource if the version that was last modified at the time in your
previous Last-Modified response header is no longer the latest one.
If-Unmodified-Since Last-Modified Send a copy of the resource only if it is the same as the version that was last modi-
fied at the time in your previous Last-Modified response header.
If-Match ETag Send a copy of the resource if its entity tag is the same as that of the one in your
previous ETag response header.
If-None-Match ETag Send a copy of the resource if its entity tag is different from that of the one in your
previous ETag response header.
Range Requests |363
HTTP groups validators into two classes: weak validators and strong validators.
Weak validators may not always uniquely identify an instance of a resource; strong
validators must. An example of a weak validator is the size of the object in bytes. The
resource content might change even thought the size remains the same, so a hypo-
thetical byte-count validator only weakly indicates a change. A cryptographic check-
sum of the contents of the resource (such as MD5), however, is a strong validator; it
changes when the document changes.
The last-modified time is considered a weak validator because, although it specifies
the time at which the resource was last modified, it specifies that time to an accuracy
of at most one second. Because a resource can change multiple times in a second,
and because servers can serve thousands of requests per second, the last-modified
date might not always reflect changes. The ETag header is considered a strong vali-
dator, because the server can place a distinct value in the ETag header every time a
value changes. Version numbers and digest checksums are good candidates for the
ETag header, but they can contain any arbitrary text. ETag headers are flexible; they
take arbitrary text values (“tags”), and can be used to devise a variety of client and
server validation strategies.
Clients and servers may sometimes want to adopt a looser version of entity-tag vali-
dation. For example, a server may want to make cosmetic changes to a large, popu-
lar cached document without triggering a mass transfer when caches revalidate. In
this case, the server might advertise a “weak” entity tag by prefixing the tag with
“W/”. A weak entity tag should change only when the associated entity changes in a
semantically significant way. A strong entity tag must change whenever the associ-
ated entity value changes in any way.
The following example shows how a client might revalidate with a server using a
weak entity tag. The server would return a body only if the content changed in a
meaningful way from Version 4.0 of the document:
GET /announce.html HTTP/1.1
If-None-Match: W/"v4.0"
In summary, when clients access the same resource more than once, they first need
to determine whether their current copy still is fresh. If it is not, they must get the lat-
est version from the server. To avoid receiving an identical copy in the event that the
resource has not changed, clients can send conditional requests to the server, specify-
ing validators that uniquely identify their current copies. Servers will then send a
copy of the resource only if it is different from the client’s copy. For more details on
cache revalidation, please refer back to “Cache Processing Steps” in Chapter 7.
Range Requests
We now understand how a client can ask a server to send it a resource only if the cli-
ent’s copy of the resource is no longer valid. HTTP goes further: it allows clients to
actually request just part or a range of a document.
364 |Chapter 15: Entities and Encodings
Imagine if you were three-fourths of the way through downloading the latest hot soft-
ware across a slow modem link, and a network glitch interrupted your connection.
You would have been waiting for a while for the download to complete, and now you
would have to start all over again, hoping the same thing does not happen again.
With range requests, an HTTP client can resume downloading an entity by asking
for the range or part of the entity it failed to get (provided that the object did not
change at the origin server between the time the client first requested it and its subse-
quent range request). For example:
GET /bigfile.html HTTP/1.1
Host: www.joes-hardware.com
Range: bytes=4000-
User-Agent: Mozilla/4.61 [en] (WinNT; I)
...
In this example, the client is requesting the remainder of the document after the first
4,000 bytes (the end bytes do not have to be specified, because the size of the docu-
ment may not be known to the requestor). Range requests of this form can be used for
a failed request where the client received the first 4,000 bytes before the failure. The
Range header also can be used to request multiple ranges (the ranges can be specified
in any order and may overlap)—for example, imagine a client connecting to multiple
servers simultaneously, requesting different ranges of the same document from differ-
ent servers in order to speed up overall download time for the document. In the case
where clients request multiple ranges in a single request, responses come back as a
single entity, with a multipart body and a Content-Type: multipart/byteranges header.
Not all servers accept range requests, but many do. Servers can advertise to clients
that they accept ranges by including the header Accept-Ranges in their responses.
The value of this header is the unit of measure, usually bytes.* For example:
HTTP/1.1 200 OK
Date: Fri, 05 Nov 1999 22:35:15 GMT
Server: Apache/1.2.4
Accept-Ranges: bytes
...
Figure 15-9 shows an example of a set of HTTP transactions involving ranges.
Range headers are used extensively by popular peer-to-peer file-sharing client software
to download different parts of multimedia files simultaneously, from different peers.
Note that range requests are a class of instance manipulations, because they are
exchanges between a client and a server for a particular instance of an object. That is,
a client’s range request makes sense only if the client and server have the same ver-
sion of a document.
* The HTTP/1.1 specification defines only the bytes token, but server and client implementors could come up
with their own units to measure or chop up an entity.
Delta Encoding |365
Delta Encoding
We have described different versions of a web page as different instances of a page. If
a client has an expired copy of a page, it requests the latest instance of the page. If
the server has a newer instance of the page, it will send it to the client, and it will
send the full new instance of the page even if only a small portion of the page actu-
ally has changed.
Rather than sending it the entire new page, the client would get the page faster if the
server sent just the changes to the client’s copy of the page (provided that the num-
ber of changes is small). Delta encoding is an extension to the HTTP protocol that
optimizes transfers by communicating changes instead of entire objects. Delta encod-
ing is a type of instance manipulation, because it relies on clients and servers
exchanging information about particular instances of an object. RFC 3229 describes
delta encoding.
Figure 15-10 illustrates more clearly the mechanism of requesting, generating, receiv-
ing, and applying a delta-encoded document. The client has to tell the server which
version of the page it has, that it is willing to accept a delta from the latest version of
page, and which algorithms it knows for applying those deltas to its current version.
Figure 15-9. Entity range request example
110001
111011
010111
000101
Client
www.joes-hardware.com
HTTP/1.1 200 OK
Content-type: text/html
Content-length: 65537
Accept-ranges: bytes
[...]
GET /bigfile.html HTTP/1.1
[...]
Request message
Response message
GET /bigfile.html HTTP.1.1
Range: bytes=20224-
[...]
Range request message
Client received only
the first 20224 bytes
of the resource
HTTP/1.1 200 OK
Range: bytes=20224-
Accept-ranges: bytes
[...]
Range response message
The client’s original request was
interrupted, but a second request
for the part of the message that
was not received allows the
client to resume from the point
of the interruption
www.joes-hardware.com
366 |Chapter 15: Entities and Encodings
The server has to check if it has the client’s version of the page and how to compute
deltas from the latest version and the client’s version (there are several algorithms for
computing the difference between two objects). It then has to compute the delta,
send it to the client, let the client know that it’s sending a delta, and specify the new
identifier for the latest version of the page (because this is the version that the client
will end up with after it applies the delta to its old version).
The client uses the unique identifier for its version of the page (sent by the server in
its previous response to the client in the ETag header) in an If-None-Match header.
This is the client’s way of telling the server, “if the latest version of the page you have
Figure 15-10. Mechanics of delta-encoding
Client
Server
HTTP/1.1 200 OK
Content-type: text/html
Expires: Mon, 01 Feb 2001 12:00:00 GMT
Etag: abcdefghi09876AF
...
GET /bigfile.html HTTP/1.1
Date: Mon, 01 Feb 2001 12:03:00 GMT
Request message
Response message
GET /bigfile.html HTTP.1.1
If-None-Match: abcdefghi09876AF
A-IM: diffe
Date: Tue, 02 Feb 2001 03:03:00 GMT
Delta request message
Client receives this response and
caches it. The next day, the client
tries to access the same page and
sees its cached copy has expired,
so it sends a request to the server
requesting the latest copy. Since it
has a cached copy, it tells the server
which copy it has and indicates
its willingness to accept a delta.
HTTP/1.1 226 IM Used
IM: diffe
Etag: zywxtuv123456BG
Delta-base: abcdefghi09876AF
...
Delta response message
Client receives the delta and applies
it to its cached version of the
page, generating the latest version
of the page. The client also updates its
ETag to that of the new version of the page.
Page on Monday
Feb 1, 2001 at 12:03 p m.
Hello, welcome to
Joe’s Hardware store.
Today’s special is on
hammers.
Page on Tuesday
Feb 2, 2001 at 03:03 a m.
Hello, welcome to
Joe’s Hardware store.
Today’s special is on
chisels.
Delta generator
5c.
chisels.
.
Delta
Delta applier
Hello, welcome to
Joe’s Hardware store.
Today’s special is on
chisels.
Delta Encoding |367
does not have this same ETag, send me the latest version of the page.” Just the If-
None-Match header, then, would cause the server to send the client the full latest
version of the page (if it was different from the client’s version).
The client can tell the server, however, that it is willing to accept a delta of the page
by also sending an A-IM header. A-IM is short for Accept-Instance-Manipulation
(“Oh, by the way, I do accept some forms of instance manipulation, so if you apply
one of those you will not have to send me the full document.”). In the A-IM header,
the client specifies the algorithms it knows how to apply in order to generate the lat-
est version of a page given an old version and a delta. The server sends back the fol-
lowing: a special response code (226 IM Used) telling the client that it is sending it
an instance manipulation of the requested object, not the full object itself; an IM
(short for Instance-Manipulation) header, which specifies the algorithm used to com-
pute the delta; the new ETag header; and a Delta-Base header, which specifies the
ETag of the document used as the base for computing the delta (ideally, the same as
the ETag in the client’s If-None-Match request!). The headers used in delta encoding
are summarized in Table 15-5.
Instance Manipulations, Delta Generators,
and Delta Appliers
Clients can specify the types of instance manipulation they accept using the A-IM
header. Servers specify the type of instance manipulation used in the IM header. Just
what are the types of instance manipulation that are accepted, and what do they do?
Table 15-6 lists some of the IANA registered types of instance manipulations.
Table 15-5. Delta-encoding headers
Header Description
ETag Unique identifier for each instance of a document. Sent by the server in the response; used by clients in sub-
sequent requests in If-Match and If-None-Match headers.
If-None-Match Request header sent by the client, asking the server for a document if and only if the client’s version of the
document is different from the server’s.
A-IM Client request header indicating types of instance manipulations accepted.
IM Server response header specifying the type of instance manipulation applied to the response. This header is
sent when the response code is 226 IM Used.
Delta-Base Server response header that specifies the ETag of the base document used for generating the delta (should
be the same as the ETag in the client request’s If-None-Match header).
Table 15-6. IANA registered types of instance manipulations
Type Description
vcdiff Delta using the vcdiff algorithma
diffe Delta using the Unix diff -e command
gdiff Delta using the gdiff algorithmb
368 |Chapter 15: Entities and Encodings
A “delta generator” at the server, as in Figure 15-10, takes the base document and
the latest instance of the document and computes the delta between the two using
the algorithm specified by the client in the A-IM header. At the client side, a “delta
applier” takes the delta and applies it to the base document to generate the latest
instance of the document. For example, if the algorithm used to generate the delta is
the Unix diff -e command, the client can apply the delta using the functionality of the
Unix ed text editor, because diff -e <file1> <file2> generates the set of ed commands
that will convert <file1> into <file2>.ed is a very simple editor with a few supported
commands. In the example in Figure 15-10, 5c says delete line 5 in the base docu-
ment, and chisels.<cr>. says add “chisels.”. That’s it. More complicated instructions
can be generated for bigger changes. The Unix diff -e algorithm does a line-by-line
comparison of files. This obviously is okay for text files but breaks down for binary
files. The vcdiff algorithm is more powerful, working even for non-text files and gen-
erally producing smaller deltas than diff -e.
The delta encoding specification defines the format of the A-IM and IM headers in
detail. Suffice it to say that multiple instance manipulations can be specified in these
headers (along with corresponding quality values). Documents can go through multi-
ple instance manipulations before being returned to clients, in order to maximize
compression. For example, deltas generated by the vcdiff algorithm may in turn be
compressed using the gzip algorithm. The server response would then contain the
header IM: vcdiff, gzip. The client would first gunzip the content, then apply the
results of the delta to its base page in order to generate the final document.
Delta encoding can reduce transfer times, but it can be tricky to implement. Imagine
a page that changes frequently and is accessed by many different people. A server
supporting delta encoding must keep all the different copies of that page as it
changes over time, in order to figure out what’s changed between any requesting cli-
ent’s copy and the latest copy. (If the document changes frequently, as different cli-
ents request the document, they will get different instances of the document. When
they make subsequent requests to the server, they will be requesting changes
between their instance of the document and the latest instance of the document. To
be able to send them just the changes, the server must keep copies of all the previous
gzip Compression using the gzip algorithm
deflate Compression using the deflate algorithm
range Used in a server response to indicate that the response is partial content as the result of a range selection
identity Used in a client request’s A-IM header to indicate that the client is willing to accept an identity instance
manipulation
aInternet draft draft-korn-vcdiff-01 describes the vcdiff algorithm. This specification was approved by the IESG in early 2002 and
should be released in RFC form shortly.
bhttp://www.w3org/TR/NOTE-gdiff-19970901.html describes the GDIFF algorithm.
Table 15-6. IANA registered types of instance manipulations (continued)
Type Description
For More Information |369
instances that the clients have.) In exchange for reduced latency in serving docu-
ments, servers need to increase disk space to keep old instances of documents
around. The extra disk space necessary to do so may quickly negate the benefits from
the smaller transfer amounts.
For More Information
For more information on entities and encodings, see:
http://www.ietf.org/rfc/rfc2616.txt
The HTTP/1.1 specification, RFC 2616, is the primary reference for entity body
management and encodings.
http://www.ietf.org/rfc/rfc3229.txt
RFC 3229, “Delta Encoding in HTTP,” describes how delta encoding can be
supported as an extension to HTTP/1.1.
Introduction to Data Compression
Khalid Sayood, Morgan Kaufmann Publishers. This book explains some of the
compression algorithms supported by HTTP content encodings.
http://www.ietf.org/rfc/rfc1521.txt
RFC 1521, “Multipurpose Internet Mail Extensions, Part One: Mechanisms for
Specifying and Describing the Format of Internet Message Bodies,” describes the
format of MIME bodies. This reference material is useful because HTTP bor-
rows heavily from MIME. In particular, this document is designed to provide
facilities to include multiple objects in a single message, to represent body text in
character sets other than US-ASCII, to represent formatted multi-font text mes-
sages, and to represent nontextual material such as images and audio fragments.
http://www.ietf.org/rfc/rfc2045.txt
RFC 2045, “Multipurpose Internet Mail Extensions, Part One: Format of Inter-
net Message Bodies,” specifies the various headers used to describe the structure
of MIME messages, many of which are similar or identical to HTTP.
http://www.ietf.org/rfc/rfc1864.txt
RFC 1864, “The Content-MD5 Header Field,” provides some historical detail
about the behavior and intended use of the Content-MD5 header field in MIME
content as a message integrity check.
http://www.ietf.org/rfc/rfc3230.txt
RFC 3230, “Instance Digests in HTTP,” describes improvements to HTTP entity-
digest handling that fix weaknesses present in the Content-MD5 formulation.
370
CHAPTER 16
Internationalization
Every day, billions of people write documents in hundreds of languages. To live up
to the vision of a truly world-wide Web, HTTP needs to support the transport and
processing of international documents, in many languages and alphabets.
This chapter covers two primary internationalization issues for the Web: character
set encodings and language tags. HTTP applications use character set encodings to
request and display text in different alphabets, and they use language tags to describe
and restrict content to languages the user understands. We finish with a brief chat
about multilingual URIs and dates.
This chapter:
• Explains how HTTP interacts with schemes and standards for multilingual
alphabets
• Gives a rapid overview of the terminology, technology, and standards to help
HTTP programmers do things right (readers familiar with character encodings
can skip this section)
• Explains the standard naming system for languages, and how standardized lan-
guage tags describe and select content
• Outlines rules and cautions for international URIs
• Briefly discusses rules for dates and other internationalization issues
HTTP Support for International Content
HTTP messages can carry content in any language, just as it can carry images, mov-
ies, or any other kind of media. To HTTP, the entity body is just a box of bits.
To support international content, servers need to tell clients about the alphabet and
languages of each document, so the client can properly unpack the document bits
into characters and properly process and present the content to the user.
Character Sets and HTTP |371
Servers tell clients about a document’s alphabet and language with the HTTP
Content-Type charset parameter and Content-Language headers. These headers
describe what’s in the entity body’s “box of bits,” how to convert the contents into
the proper characters that can be displayed onscreen, and what spoken language the
words represent.
At the same time, the client needs to tell the server which languages the user under-
stands and which alphabetic coding algorithms the browser has installed. The client
sends Accept-Charset and Accept-Language headers to tell the server which charac-
ter set encoding algorithms and languages the client understands, and which of them
are preferred.
The following HTTP Accept headers might be sent by a French speaker who prefers
his native language (but speaks some English in a pinch) and who uses a browser
that supports the iso-8859-1 West European charset encoding and the UTF-8 Uni-
code charset encoding:
Accept-Language: fr, en;q=0.8
Accept-Charset: iso-8859-1, utf-8
The parameter “q=0.8” is a quality factor, giving lower priority to English (0.8) than
to French (1.0 by default).
Character Sets and HTTP
So, let’s jump right into the most important (and confusing) aspects of web interna-
tionalization—international alphabetic scripts and their character set encodings.
Web character set standards can be pretty confusing. Lots of people get frustrated
when they first try to write international web software, because of complex and
inconsistent terminology, standards documents that you have to pay to read, and
unfamiliarity with foreign languages. This section and the next section should make
it easier for you to use character sets with HTTP.
Charset Is a Character-to-Bits Encoding
The HTTP charset values tell you how to convert from entity content bits into char-
acters in a particular alphabet. Each charset tag names an algorithm to translate bits
to characters (and vice versa). The charset tags are standardized in the MIME charac-
ter set registry, maintained by the IANA (see http://www.iana.org/assignments/
character-sets). Appendix H summarizes many of them.
The following Content-Type header tells the receiver that the content is an HTML
file, and the charset parameter tells the receiver to use the iso-8859-6 Arabic charac-
ter set decoding scheme to decode the content bits into characters:
Content-Type: text/html; charset=iso-8859-6
372 |Chapter 16: Internationalization
The iso-8859-6 encoding scheme maps 8-bit values into both the Latin and Arabic
alphabets, including numerals, punctuation and other symbols.*For example, in
Figure 16-1, the highlighted bit pattern has code value 225, which (under iso-8859-6)
maps into the Arabic letter “FEH” (a sound like the English letter “F”).
Some character encodings (e.g., UTF-8 and iso-2022-jp) are more complicated, vari-
able-length codes, where the number of bits per character varies. This type of coding
lets you use extra bits to support alphabets with large numbers of characters (such as
Chinese and Japanese), while using fewer bits to support standard Latin characters.
How Character Sets and Encodings Work
Let’s see what character sets and encodings really do.
We want to convert from bits in a document into characters that we can display
onscreen. But because there are many different alphabets, and many different ways
of encoding characters into bits (each with advantages and disadvantages), we need a
standard way to describe and apply the bits-to-character decoding algorithm.
Bits-to-character conversions happen in two steps, as shown in Figure 16-2:
• In Figure 16-2a, bits from a document are converted into a character code that
identifies a particular numbered character in a particular coded character set. In
the example, the decoded character code is numbered 225.
• In Figure 16-2b, the character code is used to select a particular element of the
coded character set. In iso-8859-6, the value 225 corresponds to “ARABIC LET-
TER FEH.” The algorithms used in Steps a and b are determined from the
MIME charset tag.
A key goal of internationalized character systems is the isolation of the semantics
(letters) from the presentation (graphical presentation forms). HTTP concerns itself
* Unlike Chinese and Japanese, Arabic has only 28 characters. Eight bits provides 256 unique values, which
gives plenty of room for Latin characters, Arabic characters, and other useful symbols.
Figure 16-1. The charset parameter tells the client how to go from bits to characters
HTTP/1.1 200 OK
Content-type: text/html; charset=iso-8859-6
Content-length: 18572
Content-language: ar
00100101110100100101001001111101
01010010100111101001111110000110
01010101011100000101010001010011
01011111001000010101111101010...
Entity body
Code bits in HTTP response
iso-8859-6 decoding
of code
11100001
(decimal 225)
Arabic letter Feh
Character
Character Sets and HTTP |373
only with transporting the character data and the associated language and charset
labels. The presentation of the character shapes is handled by the user’s graphics dis-
play software (browser, operating system, fonts), as shown in Figure 16-2c.
The Wrong Charset Gives the Wrong Characters
If the client uses the wrong charset parameter, the client will display strange, bogus
characters. Let’s say a browser got the value 225 (binary 11100001) from the body:
• If the browser thinks the body is encoded with iso-8859-1 Western European
character codes, it will show a lowercase Latin “a” with acute accent:
• If the browser is using iso-8859-6 Arabic codes, it will show “FEH”:
• If the browser is using iso-8859-7 Greek, it will show a small “Alpha”:
Figure 16-2. HTTP “charset” combines a character encoding scheme and a coded character set
65 LATIN CAPITAL LETTER A
66 LATIN CAPITAL LETTER B
224 ARABIC TATWEEL
225 ARABIC LETTER FEH
226 ARABIC LETTER QAF
227 ARABIC LETTER KAF
...11100001
Data bits
encoding scheme
(using iso-8859-6’s encoding)
225
Character code
(in iso-8859-6 set)
Coded character set
Unique character
"ARABIC LETTER FEH"
Fonts and presentation logic
Glyph
(a) Decode using encoding scheme (b) Find character using coded
character set (c) Find display shape using fonts and
formatting software
MIME charset tag describes the combination of character
encoding scheme and coded character set mapping
(iso-8859-6 coded
character set)
374 |Chapter 16: Internationalization
• If the browser is using iso-8859-8 Hebrew codes, it will show “BET”:
Standardized MIME Charset Values
The combination of a particular character encoding and a particular coded character
set is called a MIME charset. HTTP uses standardized MIME charset tags in the Con-
tent-Type and Accept-Charset headers. MIME charset values are registered with the
IANA.*Table 16-1 lists a few MIME charset encoding schemes used by documents
and browsers. A more complete list is provided in Appendix H.
* See http://www.iana.org/numbers.htm for the list of registered charset values.
Table 16-1. MIME charset encoding tags
MIME charset value Description
us-ascii The famous character encoding standardized in 1968 as ANSI_X3.4-1968. It is also named ASCII, but
the “US” prefix is preferred because of several international variants in ISO 646 that modify selected
characters. US-ASCII maps 7-bit values into 128 characters. The high bit is unused.
iso-8859-1 iso-8859-1 is an 8-bit extension to ASCII to support Western European languages. It uses the high bit
to include many West European characters, while leaving the ASCII codes (0–127) intact. Also called
iso-latin-1, or nicknamed “Latin1.”
iso-8859-2 Extends ASCII to include characters for Central and Eastern European languages, including Czech,
Polish, and Romanian. Also called iso-latin-2.
iso-8859-5 Extends ASCII to include Cyrillic characters, for languages including Russian, Serbian, and Bulgarian.
iso-8859-6 Extends ASCII to include Arabic characters. Because the shapes of Arabic characters change depend-
ing on their position in a word, Arabic requires a display engine that analyzes the context and gener-
ates the correct shape for each character.
iso-8859-7 Extends ASCII to include modern Greek characters. Formerly known as ELOT-928 or ECMA-118:1986.
iso-8859-8 Extends ASCII to include Hebrew and Yiddish characters.
iso-8859-15 Updates iso-8859-1, replacing some less-needed punctuation and fraction symbols with forgotten
French and Finnish letters and replacing the international currency sign with the symbol for the new
Euro currency. This character set is nicknamed “Latin0” and may one day replace iso-8859-1 as the
preferred default character set in Europe.
iso-2022-jp iso-2022-jp is a widely used encoding for Japanese email and web content. It is a variable-length
encoding scheme that supports ASCII characters with single bytes but uses three-character modal
escape sequences to shift into three different Japanese character sets.
euc-jp euc-jp is an ISO 2022–compliant variable-length encoding that uses explicit bit patterns to identify
each character, without requiring modes and escape sequences. It uses 1-byte, 2-byte, and 3-byte
sequences of characters to identify characters in multiple Japanese character sets.
Shift_JIS This encoding was originally developed by Microsoft and sometimes is called SJIS or MS Kanji. It is a
bit complicated, for reasons of historic compatibility, and it cannot map all characters, but it still is
common.
Character Sets and HTTP |375
Content-Type Charset Header and META Tags
Web servers send the client the MIME charset tag in the Content-Type header, using
the charset parameter:
Content-Type: text/html; charset=iso-2022-jp
If no charset is explicitly listed, the receiver may try to infer the character set from
the document contents. For HTML content, character sets might be found in
<META HTTP-EQUIV="Content-Type"> tags that describe the charset.
Example 16-1 shows how HTML META tags set the charset to the Japanese encod-
ing iso-2022-jp. If the document is not HTML, or there is no META Content-Type
tag, software may attempt to infer the character encoding by scanning the actual text
for common patterns indicative of languages and encodings.
If a client cannot infer a character encoding, it assumes iso-8859-1.
The Accept-Charset Header
There are thousands of defined character encoding and decoding methods, devel-
oped over the past several decades. Most clients do not support all the various char-
acter coding and mapping systems.
HTTP clients can tell servers precisely which character systems they support, using
the Accept-Charset request header. The Accept-Charset header value provides a list
of character encoding schemes that the client supports. For example, the following
HTTP request header indicates that a client accepts the Western European iso-8859-1
koi8-r KOI8-R is a popular 8-bit Internet character set encoding for Russian, defined in IETF RFC 1489. The
initials are transliterations of the acronym for “Code for Information Exchange, 8 bit, Russian.”
utf-8 UTF-8 is a common variable-length character encoding scheme for representing UCS (Unicode),
which is the Universal Character Set of the world’s characters. UTF-8 uses a variable-length encoding
for character code values, representing each character by from one to six bytes. One of the primary
features of UTF-8 is backward compatibility with ordinary 7-bit ASCII text.
windows-1252 Microsoft calls its coded character sets “code pages.” Windows code page 1252 (a.k.a. “CP1252” or
“WinLatin1”) is an extension of iso-8859-1.
Example 16-1. Character encoding can be specified in HTML META tags
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-2022-jp">
<META LANG="jp">
<TITLE>A Japanese Document</TITLE>
</HEAD>
<BODY>
...
Table 16-1. MIME charset encoding tags (continued)
MIME charset value Description
376 |Chapter 16: Internationalization
character system as well as the UTF-8 variable-length Unicode compatibility system.
A server is free to return content in either of these character encoding schemes.
Accept-Charset: iso-8859-1, utf-8
Note that there is no Content-Charset response header to match the Accept-Charset
request header. The response character set is carried back from the server by the
charset parameter of the Content-Type response header, to be compatible with
MIME. It’s too bad this isn’t symmetric, but all the information still is there.
Multilingual Character Encoding Primer
The previous section described how the HTTP Accept-Charset header and the
Content-Type charset parameter carry character-encoding information from the cli-
ent and server. HTTP programmers who do a lot of work with international applica-
tions and content need to have a deeper understanding of multilingual character sys-
tems to understand technical specifications and properly implement software.
It isn’t easy to learn multilingual character systems—the terminology is complex and
inconsistent, you often have to pay to read the standards documents, and you may
be unfamiliar with the other languages with which you’re working. This section is an
overview of character systems and standards. If you are already comfortable with
character encodings, or are not interested in this detail, feel free to jump ahead to
“Language Tags and HTTP.”
Character Set Terminology
Here are eight terms about electronic character systems that you should know:
Character
An alphabetic letter, numeral, punctuation mark, ideogram (as in Chinese), sym-
bol, or other textual “atom” of writing. The Universal Character Set (UCS) ini-
tiative, known informally as Unicode,*has developed a standardized set of
textual names for many characters in many languages, which often are used to
conveniently and uniquely name characters.†
Glyph
A stroke pattern or unique graphical shape that describes a character. A charac-
ter may have multiple glyphs if it can be written different ways (see Figure 16-3).
Coded character
A unique number assigned to a character so that we can work with it.
Coding space
A range of integers that we plan to use as character code values.
* Unicode is a commercial consortium based on UCS that drives commercial products.
† The names look like “LATIN CAPITAL LETTER S” and “ARABIC LETTER QAF.”
Multilingual Character Encoding Primer |377
Code width
The number of bits in each (fixed-size) character code.
Character repertoire
A particular working set of characters (a subset of all the characters in the world).
Coded character set
A set of coded characters that takes a character repertoire (a selection of charac-
ters from around the world) and assigns each character a code from a coding
space. In other words, it maps numeric character codes to real characters.
Character encoding scheme
An algorithm to encode numeric character codes into a sequence of content bits
(and to decode them back). Character encoding schemes can be used to reduce
the amount of data required to identify characters (compression), work around
transmission restrictions, and unify overlapping coded character sets.
Charset Is Poorly Named
Technically, the MIME charset tag (used in the Content-Type charset parameter and
the Accept-Charset header) doesn’t specify a character set at all. The MIME charset
value names a total algorithm for mapping data bits to codes to unique characters. It
combines the two separate concepts of character encoding scheme and coded charac-
ter set (see Figure 16-2).
This terminology is sloppy and confusing, because there already are published stan-
dards for character encoding schemes and for coded character sets.*Here’s what the
HTTP/1.1 authors say about their use of terminology (in RFC 2616):
The term “character set” is used in this document to refer to a method ... to convert a
sequence of octets into a sequence of characters... Note: This use of the term “charac-
ter set” is more commonly referred to as a “character encoding.” However, since
HTTP and MIME share the same registry, it’s important that the terminology also be
shared.
The IETF also adopts nonstandard terminology in RFC 2277:
This document uses the term “charset” to mean a set of rules for mapping from a
sequence of octets to a sequence of characters, such as the combination of a coded
character set and a character encoding scheme; this is also what is used as an identifier
in MIME “charset=” parameters, and registered in the IANA charset registry. (Note
that this is NOT a term used by other standards bodies, such as ISO).
So, be careful when reading standards documents, so you know exactly what’s being
defined. Now that we’ve got the terminology sorted out, let’s look a bit more closely
at characters, glyphs, character sets, and character encodings.
* Worse, the MIME charset tag often co-opts the name of a particular coded character set or encoding scheme.
For example, iso-8859-1 is a coded character set (it assigns numeric codes to a set of 256 European characters),
but MIME uses the charset value “iso-8859-1” to mean an 8-bit identity encoding of the coded character set.
This imprecise terminology isn’t fatal, but when reading standards documents, be clear on the assumptions.
378 |Chapter 16: Internationalization
Characters
Characters are the most basic building blocks of writing. A character represents an
alphabetic letter, numeral, punctuation mark, ideogram (as in Chinese), mathemati-
cal symbol, or other basic unit of writing.
Characters are independent of font and style. Figure 16-3 shows several variants of
the same character, called “LATIN SMALL LETTER A.” A native reader of Western
European languages would immediately recognize all five of these shapes as the same
character, even though the stroke patterns and styles are quite different.
Many writing systems also have different stroke shapes for a single character,
depending on the position of the character in the word. For example, the four
strokes in Figure 16-4 all represent the character “ARABIC LETTER AIN.”*
Figure 16-4a shows how “AIN” is written as a standalone character. Figure 16-4d
shows “AIN” at the beginning of a word, Figure 16-4c shows “AIN” in the middle of
a word, and Figure 16-4b shows “AIN” at the end of a word.†
Glyphs, Ligatures, and Presentation Forms
Don’t confuse characters with glyphs. Characters are the unique, abstract “atoms” of
language. Glyphs are the particular ways you draw each character. Each character
has many different glyphs, depending on the artistic style and script.‡
Also, don’t confuse characters with presentation forms. To make writing look
nicer, many handwritten scripts and typefaces let you join adjacent characters into
pretty ligatures, in which the two characters smoothly connect. English-speaking
Figure 16-3. One character can have many different written forms
* The sound “AIN” is pronounced something like “ayine,” but toward the back of the throat.
Figure 16-4. Four positional forms of the single character “ARABIC LETTER AIN”
† Note that Arabic words are written from right to left.
‡ Many people use the term “glyph” to mean the final rendered bitmap image, but technically a glyph is the
inherent shape of a character, independent of font and minor artistic style. This distinction isn’t very easy to
apply, or useful for our purposes.
(a) Standalone (b)Final position (c) Medial position (d) Initial postion
(These different glyphs represent the same character, “ARABIC LETTER AIN”)
Multilingual Character Encoding Primer |379
typesetters often join “F” and “I” into an “FI ligature” (see Figure 16-5a–b), and
Arabic writers often join the “LAM” and “ALIF” characters into an attractive liga-
ture (Figure 16-5c–d).
Here’s the general rule: if the meaning of the text changes when you replace one
glyph with another, the glyphs are different characters. Otherwise, they are the same
characters, with a different stylistic presentation.*
Coded Character Sets
Coded character sets, defined in RFCs 2277 and 2130, map integers to characters.
Coded character sets often are implemented as arrays,†indexed by code number (see
Figure 16-6). The array elements are characters.‡
Let’s look at a few important coded character set standards, including the historic
US-ASCII character set, the iso-8859 extensions to ASCII, the Japanese JIS X 0201
character set, and the Universal Character Set (Unicode).
US-ASCII: The mother of all character sets
ASCII is the most famous coded character set, standardized back in 1968 as ANSI
standard X3.4 “American Standard Code for Information Interchange.” ASCII uses
Figure 16-5. Ligatures are stylistic presentation forms of adjacent characters, not new characters
* The division between semantics and presentation isn’t always clear. For ease of implementation, some pre-
sentation variants of the same characters have been assigned distinct characters, but the goal is to avoid this.
† The arrays can be multidimensional, so different bits of the code number index different axes of the array.
Figure 16-6. Coded character sets can be thought of as arrays that map numeric codes to characters
‡ Figure 16-6 uses a grid to represent a coded character set. Each element of the grid contains a character
image. These images are symbolic. The presence of an image “D” is shorthand for the character “LATIN
CAPITAL LETTER D,” not for any particular graphical glyph.
(a) Without FI ligature (b) With FI ligature (c) Without LA ligature (d) With LA ligature
ALIF LAM LAM and ALIF
“LATIN CAPTIAL LETTER D”
US-ASCII coded character set
68
Code 68 (0x44)
380 |Chapter 16: Internationalization
only the code values 0–127, so only 7 bits are required to cover the code space. The
preferred name for ASCII is “US-ASCII,” to distinguish it from international variants
of the 7-bit character set.
HTTP messages (headers, URIs, etc.) use US-ASCII.
iso-8859
The iso-8859 character set standards are 8-bit supersets of US-ASCII that use the
high bit to add characters for international writing. The additional space provided by
the extra bit (128 extra codes) isn’t large enough to hold even all of the European
characters (not to mention Asian characters), so iso-8859 provides customized char-
acter sets for different regions:
iso-8859-1, also known as Latin1, is the default character set for HTML. It can be
used to represent text in most Western European languages. There has been some
discussion of replacing iso-8859-1 with iso-8859-15 as the default HTTP coded char-
acter set, because it includes the new Euro currency symbol. However, because of
the widespread adoption of iso-8859-1, it’s unlikely that a widespread change to iso-
8859-15 will be adopted for quite some time.
JIS X 0201
JIS X 0201 is an extremely minimal character set that extends ASCII with Japanese
half width katakana characters. The half-width katakana characters were originally
used in the Japanese telegraph system. JIS X 0201 is often called “JIS Roman.” JIS is
an acronym for “Japanese Industrial Standard.”
JIS X 0208 and JIS X 0212
Japanese includes thousands of characters from several writing systems. While it is
possible to limp by (painfully) using the 63 basic phonetic katakana characters in JIS
X 0201, a much more complete character set is required for practical use.
iso-8859-1 Western European languages (e.g., English, French)
iso-8859-2 Central and Eastern European languages (e.g., Czech, Polish)
iso-8859-3 Southern European languages
iso-8859-4 Northern European languages (e.g., Latvian, Lithuanian, Greenlandic)
iso-8859-5 Cyrillic (e.g., Bulgarian, Russian, Serbian)
iso-8859-6 Arabic
iso-8859-7 Greek
iso-8859-8 Hebrew
iso-8859-9 Turkish
iso-8859-10 Nordic languages (e.g., Icelandic, Inuit)
iso-8859-15 Modification to iso-8859-1 that includes the new Euro currency character
Multilingual Character Encoding Primer |381
The JIS X 0208 character set was the first multi-byte Japanese character set; it
defined 6,879 coded characters, most of which are Chinese-based kanji. The JIS X
0212 character set adds an additional 6,067 characters.
UCS
The Universal Character Set (UCS) is a worldwide standards effort to combine all of
the world’s characters into a single coded character set. UCS is defined by ISO
10646. Unicode is a commercial consortium that tracks the UCS standards. UCS has
coding space for millions of characters, although the basic set consists of only about
50,000 characters.
Character Encoding Schemes
Character encoding schemes pack character code numbers into content bits and
unpack them back into character codes at the other end (Figure 16-7). There are
three broad classes of character encoding schemes:
Fixed width
Fixed-width encodings represent each coded character with a fixed number of
bits. They are fast to process but can waste space.
Variable width (nonmodal)
Variable-width encodings use different numbers of bits for different character
code numbers. They can reduce the number of bits required for common charac-
ters, and they retain compatibility with legacy 8-bit character sets while allowing
the use of multiple bytes for international characters.
Variable width (modal)
Modal encodings use special “escape” patterns to shift between different modes.
For example, a modal encoding can be used to switch between multiple, over-
lapping character sets in the middle of text. Modal encodings are complicated to
process, but they can efficiently support complicated writing systems.
Let’s look at a few common encoding schemes.
Figure 16-7. Character encoding scheme encodes character codes into bits and back again
HTTP/1.1 200 OK
Content-type: text/html; charset=iso-2022-jp
Content-length: 4198
Content-lanuage: jp
00100101110100100101001001111101
01010010100111101001010011010010
01010101011100000101010001010011
01011111001000010101111101010...
Entity body
Character encoder Character decoder
382 |Chapter 16: Internationalization
8-bit
The 8-bit fixed-width identity encoding simply encodes each character code with its
corresponding 8-bit value. It supports only character sets with a code range of 256
characters. The iso-8859 family of character sets uses the 8-bit identity encoding.
UTF-8
UTF-8 is a popular character encoding scheme designed for UCS (UTF stands for
“UCS Transformation Format”). UTF-8 uses a nonmodal, variable-length encoding
for the character code values, where the leading bits of the first byte tell the length of
the encoded character in bytes, and any subsequent byte contains six bits of code
value (see Table 16-2).
If the first encoded byte has a high bit of 0, the length is just 1 byte, and the remain-
ing 7 bits contain the character code. This has the nice result of ASCII compatibility
(but not iso-8859 compatibility, because iso-8859 uses the high bit).
For example, character code 90 (ASCII “Z”) would be encoded as 1 byte (01011010),
while code 5073 (13-bit binary value 1001111010001) would be encoded into 3 bytes:
11100001 10001111 10010001
iso-2022-jp
iso-2022-jp is a widely used encoding for Japanese Internet documents. iso-2022-jp is
a variable-length, modal encoding, with all values less than 128 to prevent problems
with non–8-bit-clean software.
The encoding context always is set to one of four predefined character sets.*Special
“escape sequences” shift from one set to another. iso-2022-jp initially uses the US-
ASCII character set, but it can switch to the JIS X 0201 (JIS-Roman) character set or
the much larger JIS X 0208-1978 and JIS X 0208-1983 character sets using 3-byte
escape sequences.
Table 16-2. UTF-8 variable-width, nonmodal encoding
Character code bits Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6
0–7 0ccccccc -----
8–11 110ccccc 10cccccc ----
12–16 1110cccc 10cccccc 10cccccc - - -
17–21 11110ccc 10cccccc 10cccccc 10cccccc - -
22–26 111110cc 10cccccc 10cccccc 10cccccc 10cccccc -
27–31 1111110c 10cccccc 10cccccc 10cccccc 10cccccc 10cccccc
* The iso-2022-jp encoding is tightly bound to these four character sets, whereas some other encodings are
independent of the particular character set.
Multilingual Character Encoding Primer |383
The escape sequences are shown in Table 16-3. In practice, Japanese text begins with
“ESC $ @” or “ESC $ B” and ends with “ESC ( B” or “ESC ( J”.
When in the US-ASCII or JIS-Roman modes, a single byte is used per character.
When using the larger JIS X 0208 character set, two bytes are used per character
code. The encoding restricts the bytes sent to be between 33 and 126.*
euc-jp
euc-jp is another popular Japanese encoding. EUC stands for “Extended Unix
Code,” first developed to support Asian characters on Unix operating systems.
Like iso-2022-jp, the euc-jp encoding is a variable-length encoding that allows the
use of several standard Japanese character sets. But unlike iso-2022-jp, the euc-jp
encoding is not modal. There are no escape sequences to shift between modes.
euc-jp supports four coded character sets: JIS X 0201 (JIS-Roman, ASCII with a few
Japanese substitutions), JIS X 0208, half-width katakana (63 characters used in the
original Japanese telegraph system), and JIS X 0212.
One byte is used to encode JIS Roman (ASCII compatible), two bytes are used for JIS X
0208 and half-width katakana, and three bytes are used for JIS X 0212. The coding is a
bit wasteful but is simple to process.
The encoding patterns are outlined in Table 16-4.
Table 16-3. iso-2022-jp character set switching escape sequences
Escape sequence Resulting coded character set Bytes per code
ESC ( B US-ASCII 1
ESC ( J JIS X 0201-1976 (JIS Roman) 1
ESC $ @ JIS X 0208-1978 2
ESC $ B JIS X 0208-1983 2
* Though the bytes can have only 94 values (between 33 and 126), this is sufficient to cover all the characters
in the JIS X 0208 character sets, because the character sets are organized into a 94 ×94 grid of code values,
enough to cover all JIS X 0208 character codes.
Table 16-4. euc-jp encoding values
Which byte Encoding values
JIS X 0201 (94 coded characters)
1st byte 33–126
JIS X 0208 (6879 coded characters)
1st byte 161–254
2nd byte 161–254
384 |Chapter 16: Internationalization
This wraps up our survey of character sets and encodings. The next section explains
language tags and how HTTP uses language tags to target content to audiences.
Please refer to Appendix H for a detailed listing of standardized character sets.
Language Tags and HTTP
Language tags are short, standardized strings that name spoken languages.
We need standardized names, or some people will tag French documents as
“French,” others will use “Français,” others still might use “France,” and lazy people
might just use “Fra” or “F.” Standardized language tags avoid this confusion.
There are language tags for English (en), German (de), Korean (ko), and many other
languages. Language tags can describe regional variants and dialects of languages,
such as Brazilian Portuguese (pt-BR), U.S. English (en-US), and Hunan Chinese (zh-
xiang). There is even a standard language tag for Klingon (i-klingon)!
The Content-Language Header
The Content-Language entity header field describes the target audience languages for
the entity. If the content is intended primarily for a French audience, the Content-
Language header field would contain:
Content-Language: fr
The Content-Language header isn’t limited to text documents. Audio clips, movies,
and applications might all be intended for a particular language audience. Any media
type that is targeted to particular language audiences can have a Content-Language
header. In Figure 16-8, the audio file is tagged for a Navajo audience.
If the content is intended for multiple audiences, you can list multiple languages. As
suggested in the HTTP specification, a rendition of the “Treaty of Waitangi,” pre-
sented simultaneously in the original Maori and English versions, would call for:
Content-Language: mi, en
Half-width katakana (63 coded characters)
1st byte 142
2nd byte 161–223
JIS X 0212 (6067 coded characters)
1st byte 143
2nd byte 161–254
3rd byte 161–254
Table 16-4. euc-jp encoding values (continued)
Which byte Encoding values
Language Tags and HTTP |385
However, just because multiple languages are present within an entity does not mean
that it is intended for multiple linguistic audiences. A beginner’s language primer,
such as “A First Lesson in Latin,” which clearly is intended to be used by an English-
literate audience, would properly include only “en”.
The Accept-Language Header
Most of us know at least one language. HTTP lets us pass our language restrictions
and preferences along to web servers. If the web server has multiple versions of a
resource, in different languages, it can give us content in our preferred language.*
Here, a client requests Spanish content:
Accept-Language: es
You can place multiple language tags in the Accept-Language header to enumerate all
supported languages and the order of preference (left to right). Here, the client pre-
fers English but will accept Swiss German (de-CH) or other variants of German (de):
Accept-Language: en, de-CH, de
Clients use Accept-Language and Accept-Charset to request content they can under-
stand. We’ll see how this works in more detail in Chapter 17.
Types of Language Tags
Language tags have a standardized syntax, documented in RFC 3066, “Tags for the
Identification of Languages.” Language tags can be used to represent:
• General language classes (as in “es” for Spanish)
• Country-specific languages (as in “en-GB” for English in Great Britain)
• Dialects of languages (as in “no-bok” for Norwegian “Book Language”)
Figure 16-8. Content-Language header marks a “Rain Song” audio clip for Navajo speakers
* Servers also can use the Accept-Language header to generate dynamic content in the language of the user or
to select images or target language-appropriate merchandising promotions.
HTTP/1.1 200 OK
Content-type: audio/x-wav
Content-length: 289772
Content-language: i-navajo
http://www.canyonrecords.com/wav/534.wav
00100101110100100101
01010010100111101001
01010101011100000101
01011111001000011...
386 |Chapter 16: Internationalization
• Regional languages (as in “sgn-US-MA” for Martha’s Vineyard sign language)
• Standardized nonvariant languages (e.g., “i-navajo”)
• Nonstandard languages (e.g., “x-snowboarder-slang”*)
Subtags
Language tags have one or more parts, separated by hyphens, called subtags:
• The first subtag called the primary subtag. The values are standardized.
• The second subtag is optional and follows its own naming standard.
• Any trailing subtags are unregistered.
The primary subtag contains only letters (A–Z). Subsequent subtags can contain let-
ters or numbers, up to eight characters in length. An example is shown in Figure 16-9.
Capitalization
All tags are case-insensitive—the tags “en” and “eN” are equivalent. However, low-
ercasing conventionally is used to represent general languages, while uppercasing is
used to signify particular countries. For example, “fr” means all languages classified
as French, while “FR” signifies the country France.†
IANA Language Tag Registrations
The values of the first and second language subtags are defined by various standards
documents and their maintaining organizations. The IANA‡administers the list of
standard language tags, using the rules outlined in RFC 3066.
If a language tag is composed of standard country and language values, the tag doesn’t
have to be specially registered. Only those language tags that can’t be composed out
of the standard country and language values need to be registered specially with the
* Describes the unique dialect spoken by “shredders.”
Figure 16-9. Language tags are separated into subtags
† This convention is recommended by ISO standard 3166.
‡ See http://www.iana.org and RFC 2860.
sgn-US-MA
First subtag
(sign language)
Second subtag
(America)
Third subtag
(Massachusetts
regional variant)
Martha’s Vineyard sign language
Language Tags and HTTP |387
IANA.*The following sections outline the RFC 3066 standards for the first and sec-
ond subtags.
First Subtag: Namespace
The first subtag usually is a standardized language token, chosen from the ISO 639
set of language standards. But it also can be the letter “i” to identify IANA-registered
names, or “x” for private, extension names. Here are the rules:
If the first subtag has:
• Two characters, it is a language code from the ISO 639† and 639-1 standards
• Three characters, it is a language code listed in the ISO 639-2‡standard and
extensions
• The letter “i,” the language tag is explicitly IANA-registered
• The letter “x,” the language tag is a private, nonstandard, extension subtag
The ISO 639 and 639-2 names are summarized in Appendix G. A few examples are
shown here in Table 16-5.
* At the time of writing, only 21 language tags have been explicitly registered with the IANA, including Can-
tonese (“zh-yue”), New Norwegian (“no-nyn”), Luxembourgish (“i-lux”), and Klingon (“i-klingon”). The
hundreds of remaining spoken languages in use on the Internet have been composed from standard compo-
nents.
† See ISO standard 639, “Codes for the representation of names of languages.”
‡ See ISO 639-2, “Codes for the representation of names of languages—Part 2: Alpha-3 code.”
Table 16-5. Sample ISO 639 and 639-2 language codes
Language ISO 639 ISO 639-2
Arabic ar ara
Chinese zh chi/zho
Dutch nl dut/nla
English en eng
French fr fra/fre
German de deu/ger
Greek (Modern) el ell/gre
Hebrew he heb
Italian it ita
Japanese ja jpn
Korean ko kor
Norwegian no nor
Russian ru rus
Spanish es esl/spa
388 |Chapter 16: Internationalization
Second Subtag: Namespace
The second subtag usually is a standardized country token, chosen from the ISO
3166 set of country code and region standards. But it may also be another string,
which you may register with the IANA. Here are the rules:
If the second subtag has:
• Two characters, it’s a country/region defined by ISO 3166*
• Three to eight characters, it may be registered with the IANA
• One character, it is illegal
Some of the ISO 3166 country codes are shown in Table 16-6. The complete list of
country codes can be found in Appendix G.
Swedish sv sve/swe
Turkish tr tur
* The country codes AA, QM–QZ, XA–XZ and ZZ are reserved by ISO 3166 as user-assigned codes. These
must not be used to form language tags.
Table 16-6. Sample ISO 3166 country codes
Country Code
Brazil BR
Canada CA
China CN
France FR
Germany DE
Holy See (Vatican City State) VA
Hong Kong HK
India IN
Italy IT
Japan JP
Lebanon LB
Mexico MX
Pakistan PK
Russian Federation RU
United Kingdom GB
United States US
Table 16-5. Sample ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
Internationalized URIs |389
Remaining Subtags: Namespace
There are no rules for the third and following subtags, apart from being up to eight
characters (letters and digits).
Configuring Language Preferences
You can configure language preferences in your browser profile.
Netscape Navigator lets you set language preferences through Edit ➝Preferences...
➝Languages..., and Microsoft Internet Explorer lets you set languages through
Tools ➝ Internet Options... ➝ Languages.
Language Tag Reference Tables
Appendix G contains convenient reference tables for language tags:
• IANA-registered language tags are shown in Table G-1.
• ISO 639 language codes are shown in Table G-2.
• ISO 3166 country codes are shown in Table G-3.
Internationalized URIs
Today, URIs don’t provide much support for internationalization. With a few
(poorly defined) exceptions, today’s URIs are comprised of a subset of US-ASCII
characters. There are efforts underway that might let us include a richer set of char-
acters in the hostnames and paths of URLs, but right now, these standards have not
been widely accepted or deployed. Let’s review today’s practice.
Global Transcribability Versus Meaningful Characters
The URI designers wanted everyone around the world to be able to share URIs with
each other—by email, by phone, by billboard, even over the radio. And they wanted
URIs to be easy to use and remember. These two goals are in conflict.
To make it easy for folks around the globe to enter, manipulate, and share URIs, the
designers chose a very limited set of common characters for URIs (basic Latin alpha-
bet letters, digits, and a few special characters). This small repertoire of characters is
supported by most software and keyboards around the world.
Unfortunately, by restricting the character set, the URI designers made it much
harder for people around the globe to create URIs that are easy to use and remem-
ber. The majority of world citizens don’t even recognize the Latin alphabet, making
it nearly impossible to remember URIs as abstract patterns.
390 |Chapter 16: Internationalization
The URI authors felt it was more important to ensure transcribability and sharability
of resource identifiers than to have them consist of the most meaningful characters. So
we have URIs that (today) essentially consist of a restricted subset of ASCII characters.
URI Character Repertoire
The subset of US-ASCII characters permitted in URIs can be divided into reserved,
unreserved, and escape character classes. The unreserved character classes can be
used generally within any component of URIs that allow them. The reserved charac-
ters have special meanings in many URIs, so they shouldn’t be used in general. See
Table 16-7 for a list of the unreserved, reserved, and escape characters.
Escaping and Unescaping
URI “escapes” provide a way to safely insert reserved characters and other unsup-
ported characters (such as spaces) inside URIs. An escape is a three-character
sequence, consisting of a percent character (%) followed by two hexadecimal digit
characters. The two hex digits represent the code for a US-ASCII character.
For example, to insert a space (ASCII 32) in a URL, you could use the escape “%20”,
because 20 is the hexadecimal representation of 32. Similarly, if you wanted to
include a percent sign and have it not be treated as an escape, you could enter
“%25”, where 25 is the hexadecimal value of the ASCII code for percent.
Figure 16-10 shows how the conceptual characters for a URI are turned into code
bytes for the characters, in the current character set. When the URI is needed for
processing, the escapes are undone, yielding the underlying ASCII code bytes.
Internally, HTTP applications should transport and forward URIs with the escapes
in place. HTTP applications should unescape the URIs only when the data is needed.
And, more importantly, the applications should ensure that no URI ever is unes-
caped twice, because percent signs that might have been encoded in an escape will
themselves be unescaped, leading to loss of data.
Escaping International Characters
Note that escape values should be in the range of US-ASCII codes (0–127). Some
applications attempt to use escape values to represent iso-8859-1 extended charac-
ters (128–255)—for example, web servers might erroneously use escapes to code
Table 16-7. URI character syntax
Character class Character repertoire
Unreserved [A-Za-z0-9] | “-” | “_” | “.” | “!” | “~” | “*” | “'” | “(” | “)”
Reserved “;” | “/” | “?” | “:” | “@” | “&” | “=” | “+” | “$” | “,”
Escape “%” <HEX> <HEX>
Internationalized URIs |391
filenames that contain international characters. This is incorrect and may cause
problems with some applications.
For example, the filename Sven Ölssen.html (containing an umlaut) might be
encoded by a web server as Sven%20%D6lssen.html. It’s fine to encode the space
with %20, but is technically illegal to encode the Ö with %D6, because the code D6
(decimal 214) falls outside the range of ASCII. ASCII defines only codes up to 0x7F
(decimal 127).
Modal Switches in URIs
Some URIs also use sequences of ASCII characters to represent characters in other
character sets. For example, iso-2022-jp encoding might be used to insert “ESC ( J”
to shift into JIS-Roman and “ESC ( B” to shift back to ASCII. This works in some
local circumstances, but the behavior is not well defined, and there is no standard-
ized scheme to identify the particular encoding used for the URL. As the authors of
RFC 2396 say:
For original character sequences that contain non-ASCII characters, however, the situ-
ation is more difficult. Internet protocols that transmit octet sequences intended to
represent character sequences are expected to provide some way of identifying the
charset used, if there might be more than one [RFC2277].
However, there is currently no provision within the generic URI syntax to accomplish
this identification. An individual URI scheme may require a single charset, define a
default charset, or provide a way to indicate the charset used. It is expected that a sys-
tematic treatment of character encoding within URI will be developed as a future mod-
ification of this specification.
Currently, URIs are not very international-friendly. The goal of URI portability out-
weighed the goal of language flexibility. There are efforts currently underway to
internationalize URIs, but in the near term, HTTP applications should stick with
ASCII. It’s been around since 1968, so it can’t be all that bad.
Figure 16-10. URI characters are transported as escaped code bytes but processed unescaped
Big Sale at Joe’s
Big Sale at Joe’s
http://www.joes-hardware.com/big%20sale.txt
...
o=111
m=109
/=47
b=98
i=105
g=103
%=37
2=50
0=48
s=115
...
External form
(email, web, billboard, radio)
What you enter and send
(in current character set)
...
111
109
47
98
105
103
32
115
...
What you process
(in US-ASCII character set)
Conceptual characters URI code bytes Unescaped ASCII code byte
392 |Chapter 16: Internationalization
Other Considerations
This section discusses a few other things you should keep in mind when writing
international HTTP applications.
Headers and Out-of-Spec Data
HTTP headers must consist of characters from the US-ASCII character set. How-
ever, not all clients and servers implement this correctly, so you may on occasion
receive illegal characters with code values larger than 127.
Many HTTP applications use operating-system and library routines for processing
characters (for example, the Unix ctype character classification library). Not all of
these libraries support character codes outside of the ASCII range (0–127).
In some circumstances (generally, with older implementations), these libraries may
return improper results or crash the application when given non-ASCII characters.
Carefully read the documentation for your character classification libraries before
using them to process HTTP messages, in case the messages contain illegal data.
Dates
The HTTP specification clearly defines the legal GMT date formats, but be aware
that not all web servers and clients follow the rules. For example, we have seen web
servers send invalid HTTP Date headers with months expressed in local languages.
HTTP applications should attempt to be tolerant of out-of-spec dates, and not crash
on receipt, but they may not always be able to interpret all dates sent. If the date is
not parseable, servers should treat it conservatively.
Domain Names
DNS doesn’t currently support international characters in domain names. There are
standards efforts under way to support multilingual domain names, but they have
not yet been widely deployed.
For More Information
The very success of the World Wide Web means that HTTP applications will con-
tinue to exchange more and more content in different languages and character sets.
For more information on the important but slightly complex topic of multilingual
multimedia, please refer to the following sources.
For More Information |393
Appendixes
• IANA-registered charset tags are listed in Table H-1.
• IANA-registered language tags are shown in Table G-1.
• ISO 639 language codes are shown in Table G-2.
• ISO 3166 country codes are shown in Table G-3.
Internet Internationalization
http://www.w3.org/International/
“Making the WWW Truly World Wide”—the W3C Internationalization and
Localization web site.
http://www.ietf.org/rfc/rfc2396.txt
RFC 2396, “Uniform Resource Identifiers (URI): Generic Syntax,” is the defin-
ing document of URIs. This document includes sections describing character set
restrictions for international URIs.
CJKV Information Processing
Ken Lunde, O’Reilly & Associates, Inc. CJKV is the bible of Asian electronic
character processing. Asian character sets are varied and complex, but this book
provides an excellent introduction to the standards technologies for large charac-
ter sets.
http://www.ietf.org/rfc/rfc2277.txt
RFC 2277, “IETF Policy on Character Sets and Languages,” documents the cur-
rent policies being applied by the Internet Engineering Steering Group (IESG)
toward the standardization efforts in the Internet Engineering Task Force (IETF)
in order to help Internet protocols interchange data in multiple languages and
characters.
International Standards
http://www.iana.org/numbers.htm
The Internet Assigned Numbers Authority (IANA) contains repositories of regis-
tered names and numbers. The “Protocol Numbers and Assignments Directory”
contains records of registered character sets for use on the Internet. Because
much work on international communications falls under the domain of the ISO,
and not the Internet community, the IANA listings are not exhaustive.
http://www.ietf.org/rfc/rfc3066.txt
RFC 3066, “Tags for the Identification of Languages,” describes language tags,
their values, and how to construct them.
394 |Chapter 16: Internationalization
“Codes for the representation of names of languages”
ISO 639:1988 (E/F), The International Organization for Standardization, first
edition.
“Codes for the representation of names of languages—Part 2: Alpha-3 code”
ISO 639-2:1998, Joint Working Group of ISO TC46/SC4 and ISO TC37/SC2,
first edition.
“Codes for the representation of names of countries”
ISO 3166:1988 (E/F), The International Organization for Standardization, third
edition.
395
CHAPTER 17
Content Negotiation and Transcoding
Often, a single URL may need to correspond to different resources. Take the case of
a web site that wants to offer its content in multiple languages. If a site such as Joe’s
Hardware has both French- and English-speaking users, it might want to offer its
web site in both languages. However, if this is the case, when one of Joe’s customers
requests “http://www.joes-hardware.com,” which version should the server send?
French or English?
Ideally, the server will send the English version to an English speaker and the French
version to a French speaker—a user could go to Joe’s Hardware’s home page and get
content in the language he speaks. Fortunately, HTTP provides content-negotiation
methods that allow clients and servers to make just such determinations. Using these
methods, a single URL can correspond to different resources (e.g., a French and
English version of the same web page). These different versions are called variants.
Servers also can make other types of decisions about what content is best to send to a
client for a particular URL. In some cases, servers even can automatically generate
customized pages—for instance, a server can convert an HTML page into a WML
page for your handheld device. These kinds of dynamic content transformations are
called transcodings. They are done in response to content negotiation between HTTP
clients and servers.
In this chapter, we will discuss content negotiation and how web applications go
about their content-negotiation duties.
Content-Negotiation Techniques
There are three distinct methods for deciding which page at a server is the right one
for a client: present the choice to the client, decide automatically at the server, or ask
an intermediary to select. These three techniques are called client-driven negotiation,
server-driven negotiation, and transparent negotiation, respectively (see Table 17-1).
396 |Chapter 17: Content Negotiation and Transcoding
In this chapter, we will look at the mechanics of each technique as well as their
advantages and disadvantages.
Client-Driven Negotiation
The easiest thing for a server to do when it receives a client request is to send back a
response listing the available pages and let the client decide which one it wants to
see. This, of course, is the easiest to implement at the server and is likely to result in
the best copy being selected (provided that the list has enough information to allow
the client to pick the right copy). The disadvantage is that two requests are needed
for each page—one to get the list and a second to get the selected copy. This is a
slow and tedious process, and it’s likely to become annoying to the client.
Mechanically, there are actually two ways for servers to present the choices to the cli-
ent for selection: by sending back an HTML document with links to the different ver-
sions of the page and descriptions of each of the versions, or by sending back an
HTTP/1.1 response with the 300 Multiple Choices response code. The client
browser may receive this response and display a page with the links, as in the first
method, or it may pop up a dialog window asking the user to make a selection. In
any case, the decision is made manually at the client side by the browser user.
In addition to the increased latency and annoyance of multiple requests per page,
this method has another drawback: it requires multiple URLs—one for the main
page and one for each specific page. So, if the original request was for www.joes-
hardware.com, Joe’s server may respond with a page that has links to www.joes-
hardware.com/english and www.joes-hardware.com/french. Should clients now book-
mark the original main page or the selected ones? Should they tell their friends
about the great web site at www.joes-hardware.com or tell only their English-speak-
ing friends about the web site at www.joes-hardware.com/english?
Table 17-1. Summary of content-negotiation techniques
Technique How it works Advantages Drawbacks
Client-driven Client makes a request,
server sends list of choices
to client, client chooses.
Easiest to implement at server side. Client can
make best choice.
Adds latency: at least two
requests are needed to
get the correct content.
Server-driven Server examines client’s
request headers and
decides what version to
serve.
Quicker than client-driven negotiation. HTTP
provides a q-value mechanism to allow serv-
ers to make approximate matches and a Vary
header for servers to tell downstream devices
how to evaluate requests.
If the decision is not obvi-
ous (headers don’t match
up), the server must
guess.
Transparent An intermediate device
(usually a proxy cache)
does the request negotia-
tion on the client’s behalf.
Offloads the negotiation from the web server.
Quicker than client-driven negotiation.
No formal specifications
for how to do transparent
negotiation.
Server-Driven Negotiation |397
Server-Driven Negotiation
Client-driven negotiation has several drawbacks, as discussed in the previous sec-
tion. Most of these drawbacks center around the increased communication between
the client and server to decide on the best page in response to a request. One way to
reduce this extra communication is to let the server decide which page to send
back—but to do this, the client must send enough information about its preferences
to allow the server to make an informed decision. The server gets this information
from the client’s request headers.
There are two mechanisms that HTTP servers use to evaluate the proper response to
send to a client:
• Examining the set of content-negotiation headers. The server looks at the client’s
Accept headers and tries to match them with corresponding response headers.
• Varying on other (non–content-negotiation) headers. For example, the server
could send responses based on the client’s User-Agent header.
These two mechanisms are explained in more detail in the following sections.
Content-Negotiation Headers
Clients may send their preference information using the set of HTTP headers listed
in Table 17-2.
Notice how similar these headers are to the entity headers discussed in Chapter 15.
However, there is a clear distinction between the purposes of the two types of head-
ers. As mentioned in Chapter 15, entity headers are like shipping labels—they spec-
ify attributes of the message body that are necessary during the transfer of messages
from the server to the client. Content-negotiation headers, on the other hand, are
used by clients and servers to exchange preference information and to help choose
between different versions of a document, so that the one most closely matching the
client’s preferences is served.
Servers match clients’ Accept headers with the corresponding entity headers, listed in
Table 17-3.
Table 17-2. Accept headers
Header Description
Accept Used to tell the server what media types are okay to send
Accept-Language Used to tell the server what languages are okay to send
Accept-Charset Used to tell the server what charsets are okay to send
Accept-Encoding Used to tell the server what encodings are okay to send
398 |Chapter 17: Content Negotiation and Transcoding
Note that because HTTP is a stateless protocol (meaning that servers do not keep
track of client preferences across requests), clients must send their preference infor-
mation with every request.
If both clients sent Accept-Language header information specifying the language in
which they were interested, the server could decide which copy of www.joes-hard-
ware.com to send back to each client. Letting the server automatically pick which
document to send back reduces the latency associated with the back-and-forth com-
munication required by the client-driven model.
However, say that one of the clients prefers Spanish. Which version of the page
should the server send back? English or French? The server has just two choices:
either guess, or fall back on the client-driven model and ask the client to choose.
However, if the Spaniard happens to understand some English, he might choose the
English page—it wouldn’t be ideal, but it would do. In this case, the Spaniard needs
the ability to pass on more information about his preferences, conveying that he does
have minimal knowledge of English and that, in a pinch, English will suffice.
Fortunately, HTTP does provide a mechanism for letting clients like our Spaniard
give richer descriptions of their preferences, using quality values (“q values” for short).
Content-Negotiation Header Quality Values
The HTTP protocol defines quality values to allow clients to list multiple choices for
each category of preference and associate an order of preference with each choice.
For example, clients can send an Accept-Language header of the form:
Accept-Language: en;q=0.5, fr;q=0.0, nl;q=1.0, tr;q=0.0
Where the q values can range from 0.0 to 1.0 (with 0.0 being the lowest preference
and 1.0 being the highest). The header above, then, says that the client prefers to
receive a Dutch (nl) version of the document, but an English (en) version will do.
Under no circumstances does the client want a French (fr) or Turkish (tr) version,
though. Note that the order in which the preferences are listed is not important; only
the q values associated with them are.
Occasionally, the server may not have any documents that match any of the client’s
preferences. In this case, the server may change or transcode the document to match
the client’s preferences. This mechanism is discussed later in this chapter.
Table 17-3. Accept and matching document headers
Accept header Entity header
Accept Content-Type
Accept-Language Content-Language
Accept-Charset Content-Type
Accept-Encoding Content-Encoding
Server-Driven Negotiation |399
Varying on Other Headers
Servers also can attempt to match up responses with other client request headers,
such as User-Agent. Servers may know that old versions of a browser do not support
JavaScript, for example, and may therefore send back a version of the page that does
not contain JavaScript.
In this case, there is no q-value mechanism to look for approximate “best” matches.
The server either looks for an exact match or simply serves whatever it has (depend-
ing on the implementation of the server).
Because caches must attempt to serve correct “best” versions of cached documents,
the HTTP protocol defines a Vary header that the server sends in responses; the Vary
header tells caches (and clients, and any downstream proxies) which headers the
server is using to determine the best version of the response to send. The Vary header
is discussed in more detail later in this chapter.
Content Negotiation on Apache
Here is an overview of how the Apache web server supports content negotiation. It is
up to the web site content provider—Joe, for example—to provide different versions
of Joe’s index page. Joe must put all his index page files in the appropriate directory
on the Apache server corresponding to his web site. There are two ways to enable
content negotiation:
• In the web site directory, create a type-map file for each URI in the web site that
has variants. The type-map file lists all the variants and the content-negotiation
headers to which they correspond.
• Enable the MultiViews directive, which causes Apache to create type-map files
for the directory automatically.
Using type-map files
The Apache server needs to know what type-map files look like. To configure this,
set a handler in the server configuration file that specifies the file suffix for type-map
files. For example:
AddHandler type-map .var
This line indicates that files with the extension .var are type-map files.
Here is a sample type-map file:
URI: joes-hardware.html
URI: joes-hardware.en.html
Content-type: text/html
Content-language: en
400 |Chapter 17: Content Negotiation and Transcoding
URI: joes-hardware.fr.de.html
Content-type: text/html;charset=iso-8859-2
Content-language: fr, de
From this type-map file, the Apache server knows to send joes-hardware.en.html to
clients requesting English and joes-hardware.fr.de.html to clients requesting French.
Quality values also are supported; see the Apache server documentation.
Using MultiViews
To use MultiViews, you must enable it for the directory containing the web site, using
an Options directive in the appropriate section of the access.conf file (<Directory>,
<Location>, or <Files>).
If MultiViews is enabled and a browser requests a resource named joes-hardware, the
server looks for all files with “joes-hardware” in the name and creates a type-map file
for them. Based on the names, the server guesses the appropriate content-negotiation
headers to which the files correspond. For example, a French-language version of
joes-hardware should contain .fr.
Server-Side Extensions
Another way to implement content negotiation at the server is by server-side exten-
sions, such as Microsoft’s Active Server Pages (ASP). See Chapter 8 for an overview
of server-side extensions.
Transparent Negotiation
Transparent negotiation seeks to move the load of server-driven negotiation away
from the server, while minimizing message exchanges with the client by having an
intermediary proxy negotiate on behalf of the client. The proxy is assumed to have
knowledge of the client’s expectations and be capable of performing the negotia-
tions on its behalf (the proxy has received the client’s expectations in the request for
content). To support transparent content negotiation, the server must be able to tell
proxies what request headers the server examines to determine the best match for the
client’s request. The HTTP/1.1 specification does not define any mechanisms for
transparent negotiation, but it does define the Vary header. Servers send Vary head-
ers in their responses to tell intermediaries what request headers they use for content
negotiation.
Caching proxies can store different copies of documents accessed via a single URL. If
servers communicate their decision-making processes to caches, the caches can nego-
tiate with clients on behalf of the servers. Caches also are great places to transcode
content, because a general-purpose transcoder deployed in a cache can transcode
content from any server, not just one. Transcoding of content at a cache is illus-
trated in Figure 17-3 and discussed in more detail later in the chapter.
Transparent Negotiation |401
Caching and Alternates
Caching of content assumes that the content can be reused later. However, caches
must employ much of the decision-making logic that servers do when sending back a
response, to ensure that they send back the correct cached response to a client request.
The previous section described the Accept headers sent by clients and the correspond-
ing entity headers that servers match them up against in order to choose the best
response to each request. Caches must use these same headers to decide which cached
response to send back.
Figure 17-1 illustrates both a correct and incorrect sequence of operations involving
a cache. The first request results in the cache forwarding the request to the server
and storing the response. The second response is looked up by the cache, and a doc-
ument matching the URL is found. This document, however, is in French, and the
requestor wants a Spanish document. If the cache just sends back the French docu-
ment to the requestor, it will be behaving incorrectly.
The cache must therefore forward the second request to the server as well, and store
both the response and an “alternate” response for that URL. The cache now has two
Figure 17-1. Caches use content-negotiation headers to send back correct responses to clients
French-speaking
user
GET / HTTP/1.1
Host: www.joes-hardware.com
User-agent: spiffy multimedia browser
Accept-language: fr;q=1.0
Web server
Cache
Bonjour
Hi! Welcome to
Joe's Hardware
Store.
Hola! Bienvenido
a Joe's Hardware
Store.
Bonjour!
Bienvenue a Joe's
Hardware Store
Spanish-speaking
user
GET / HTTP/1.1
Host: www.joes-hardware.com
User-agent: spiffy multimedia browser
Accept-language: es;q=1.0
Web server
Cache
Hola! Bienvenido
a Joe's Hardware
Store.
Bonjour
Bienvenido
402 |Chapter 17: Content Negotiation and Transcoding
different documents for the same URL, just as the server does. These different ver-
sions are called variants or alternates. Content negotiation can be thought of as the
process of selecting, from the variants, the best match for a client request.
The Vary Header
Here’s a typical set of request and response headers from a browser and server:
GET http://www.joes-hardware.com/ HTTP/1.0
Proxy-Connection: Keep-Alive
User-Agent: Mozilla/4.73 [en] (WinNT; U)
Host: www.joes-hardware.com
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*
Accept-Encoding: gzip
Accept-Language: en, pdf
Accept-Charset: iso-8859-1, *, utf-8
HTTP/1.1 200 OK
Date: Sun, 10 Dec 2000 22:13:40 GMT
Server: Apache/1.3.12 OpenSSL/0.9.5a (Unix) FrontPage/4.0.4.3
Last-Modified: Fri, 05 May 2000 04:42:52 GMT
Etag: "1b7ddf-48-3912514c"
Accept-Ranges: Bytes
Content-Length: 72
Connection: close
Content-Type: text/html
What happens, however, if the server’s decision was based on headers other than the
Accept headers, such as the User-Agent header? This is not as radical as it may
sound. Servers may know that old versions of a browser do not support JavaScript,
for example, and may therefore send back a version of the page that does not have
JavaScript in it. If servers are using other headers to make their decisions about
which pages to send back, caches must know what those headers are, so that they
can perform parallel logic in choosing which cached page to send back.
The HTTP Vary response header lists all of the client request headers that the server
considers to select the document or generate custom content (in addition to the regu-
lar content-negotiation headers). For example, if the served document depends on the
User-Agent header, the Vary header must include “User-Agent”.
When a new request arrives, the cache finds the best match using the content-negoti-
ation headers. Before it can serve this document to the client, however, it must see
whether the server sent a Vary header in the cached response. If a Vary header is
present, the header values for the headers in the new request must match the header
values in the old, cached request. Because servers may vary their responses based on
client request headers, caches must store both the client request headers and the cor-
responding server response headers with each cached varaint, in order to implement
transparent negotiation. This is illustrated in Figure 17-2.
Transcoding |403
If a server’s Vary header looked like this, the huge number of different User-Agent
and Cookie values could generate many variants:
Vary: User-Agent, Cookie
A cache would have to store each document version corresponding to each variant.
When the cache does a lookup, it first does content matching with the content-nego-
tiation headers, then matches the request’s variant with cached variants. If there is no
match, the cache fetches the document from the origin server.
Transcoding
We have discussed in some detail the mechanism by which clients and servers can
choose between a set of documents for a URL and send the one that best matches the
Figure 17-2. If servers vary on specific request headers, caches must match those request headers
in addition to the regular content-negotiation headers before sending back cached responses
French-speaking
user 1
GET / HTTP/1.1
Host: www.joes-hardware.com
User-agent: spiffy multimedia browser
Accept-language: fr;q=1.0
Web server
Cache
Bonjour
I need to send her a French document.
Since she has such a cool browser, I'll
send her a media-rich version of
the page.
HTTP/1.1 200 OK
Content-language: fr
Vary: User-agent
Bonjour
[...media-rich content]
French-speaking
user 2
GET / HTTP/1.1
Host: www.joes-hardware.com
User-agent: wimpy wireless device
Accept-language: fr;q=1.0
Cache
Bonjour
HTTP/1.1 200 OK
Content-language: fr
Vary: User-agent
Bonjour
[...simple text content]
Bonjour
He wants a French copy of the document
and I have it in my cache, but I’d better
not send it to him. The server said my
cached copy was for a spiffy browser. This
guy has a wimpy wireless one. I had
better ask the server for a French version
for the wireless browser.
Web server
404 |Chapter 17: Content Negotiation and Transcoding
client’s needs. These mechanisms rely on the presence of documents that match the
client’s needs—whether they match the needs perfectly or not so well.
What happens, however, when a server does not have a document that matches the
client’s needs at all? The server may have to respond with an error, but theoretically,
the server may be able to transform one of its existing documents into something
that the client can use. This option is called transcoding.
Table 17-4 lists some hypothetical transcodings.
There are three categories of transcoding: format conversion, information synthesis,
and content injection.
Format Conversion
Format conversion is the transformation of data from one format to another to make it
viewable by a client. A wireless device seeking to access a document typically viewed
by a desktop client may be able do so with an HTML-to-WML conversion. A client
accessing a web page over a slow link that is not very interested in high-resolution
images may be able to view an image-rich page more easily if the images are reduced
in size and resolution by converting them from color to black and white and shrink-
ing them.
Format conversion is driven by the content-negotiation headers listed in Table 17-2,
although it may also be driven by the User-Agent header. Note that content transfor-
mation or transcoding is different from content encoding or transfer encoding, in
that the latter two typically are used for more efficient or safe transport of content,
whereas the former is used to make content viewable on the access device.
Information Synthesis
The extraction of key pieces of information from a document—known as informa-
tion synthesis—can be a useful transcoding process. A simple example of this is the
generation of an outline of a document based on section headings, or the removal of
advertisements and logos from a page.
Table 17-4. Hypothetical transcodings
Before After
HTML document WML document
High-resolution image Low-resolution image
Image in 64K colors Black-and-white image
Complex page with frames Simple text page without frames or images
HTML page with Java applets HTML page without Java applets
Page with ads Page with ads removed
Next Steps |405
More sophisticated technologies that categorize pages based on keywords in content
also are useful in summarizing the essence of a document. This technology often is
used by automatic web page–classification systems, such as web-page directories at
portal sites.
Content Injection
The two categories of transcodings described so far typically reduce the amount of
content in web documents, but there is another category of transformations that
increases the amount of content: content-injection transcodings. Examples of content-
injection transcodings are automatic ad generators and user-tracking systems.
Imagine the appeal (and offence) of an ad-insertion transcoder that automatically
adds advertisements to each HTML page as it goes by. Transcoding of this type has to
be dynamic—it must be done on the fly in order to be effective in adding ads that cur-
rently are relevant or somehow have been targeted for a particular user. User-tracking
systems also can be built to add content to pages dynamically, for the purpose of col-
lecting statistics about how the page is viewed and how clients surf the Web.
Transcoding Versus Static Pregeneration
An alternative to transcodings is to build different copies of web pages at the web
server—for example, one with HTML, one with WML, one with high-resolution
images, one with low-resolution images, one with multimedia content, and one with-
out. This, however, is not a very practical technique, for many reasons: any small
change in a page requires multiple pages to be modified, more space is necessary to
store all the different versions of each page, and it’s harder to catalog pages and pro-
gram web servers to serve the right ones. Some transcodings, such as ad insertion
(especially targeted ad insertion), cannot be done statically—the ad inserted will
depend upon the user requesting the page.
An on-the-fly transformation of a single root page can be an easier solution than static
pregeneration. It can come, however, at the cost of increased latency in serving the
content. Some of this computation can, however, be done by a third party, thereby off-
loading the computation from the web server—the transformation can be done by an
external agent at a proxy or cache. Figure 17-3 illustrates transcoding at a proxy cache.
Next Steps
The story of content negotiation does not end with the Accept and Content headers,
for a couple of reasons:
• Content negotiation in HTTP incurs some performance limits. Searching through
many variants for appropriate content, or trying to “guess” the best match, can
406 |Chapter 17: Content Negotiation and Transcoding
be costly. Are there ways to streamline and focus the content-negotiation proto-
col? RFCs 2295 and 2296 attempt to address this question for transparent HTTP
content negotiation.
• HTTP is not the only protocol that needs to do content negotiation. Streaming
media and fax are two other examples where client and server need to discuss
the best answer to the client’s request. Can a general content-negotiation proto-
col be developed on top of TCP/IP application protocols? The Content Negotia-
tion Working Group was formed to tackle this question. The group is now
closed, but it contributed several RFCs. See the next section for a link to the
group’s web site.
For More Information
The following Internet drafts and online documentation can give you more details
about content negotiation:
http://www.ietf.org/rfc/rfc2616.txt
RFC 2616, “Hypertext Transfer Protocol—HTTP/1.1,” is the official specifica-
tion for HTTP/1.1, the current version of the HTTP protocol. The specification
is a well-written, well-organized, detailed reference for HTTP, but it isn’t ideal
for readers who want to learn the underlying concepts and motivations of HTTP
or the differences between theory and practice. We hope that this book fills in
the underlying concepts, so you can make better use of the specification.
Figure 17-3. Content transformation or transcoding at a proxy cache
French-speaking
user
GET / HTTP/1.1
Host: www.joes-hardware.com
User-agent: wimpy wireless device
Accept-language: fr;q=1.0
Web server
Cache
Bonjour
HTTP/1.1 200 OK
Content-language: fr
Vary: User-agent
Bonjour
[...simple text content]
Bonjour Transmogrifier
I have a French copy of the document
that he wants, but my copy is very media-
rich and he has a wimpy wireless browser.
I will strip out all of the multimedia content
and send it to him.
Since I have transformed this
document for a wireless device,
I will store the transformed
copy as an alternate in case
someone else wants it as well.
For More Information |407
http://search.ietf.org/rfc/rfc2295.txt
RFC 2295, “Transparent Content Negotiation in HTTP,” is a memo describing a
transparent content-negotiation protocol on top of HTTP. The status of this
memo remains experimental.
http://search.ietf.org/rfc/rfc2296.txt
RFC 2296, “HTTP Remote Variant Selection Algorithm—RVSA 1.0,” is a memo
describing an algorithm for the transparent selection of the “best” content for a
particular HTTP request. The status of this memo remains experimental.
http://search.ietf.org/rfc/rfc2936.txt
RFC 2936, “HTTP MIME Type Handler Detection,” is a memo describing an
approach for determining the actual MIME type handlers that a browser sup-
ports. This approach can help if the Accept header is not specific enough.
http://www.imc.org/ietf-medfree/index.htm
This is a link to the Content Negotiation (CONNEG) Working Group, which
looked into transparent content negotiation for HTTP, fax, and print. This
group is now closed.
PART V
Content Publishing
and Distribution
Part V talks all about the technology for publishing and disseminating web content:
• Chapter 18, Web Hosting, discusses the ways people deploy servers in modern
web hosting environments, HTTP support for virtual web hosting, and how to
replicate content across geographically distant servers.
• Chapter 19, Publishing Systems, discusses the technologies for creating web con-
tent and installing it onto web servers.
• Chapter 20, Redirection and Load Balancing, surveys the tools and techniques for
distributing incoming web traffic among a collection of servers.
• Chapter 21, Logging and Usage Tracking, covers log formats and common
questions.
411
CHAPTER 18
Web Hosting
When you place resources on a public web server, you make them available to the
Internet community. These resources can be as simple as text files or images, or as
complicated as real-time driving maps or e-commerce shopping gateways. It’s criti-
cal that this rich variety of resources, owned by different organizations, can be conve-
niently published to web sites and placed on web servers that offer good performance
at a fair price.
The collective duties of storing, brokering, and administering content resources is
called web hosting. Hosting is one of the primary functions of a web server. You need
a server to hold, serve, log access to, and administer your content. If you don’t want
to manage the required hardware and software yourself, you need a hosting service,
or hoster. Hosters rent you serving and web-site administration services and provide
various degrees of security, reporting, and ease of use. Hosters typically pool web
sites on heavy-duty web servers for cost-efficiency, reliability, and performance.
This chapter explains some of the most important features of web hosting services
and how they interact with HTTP applications. In particular, this chapter covers:
• How different web sites can be “virtually hosted” on the same server, and how
this affects HTTP
• How to make web sites more reliable under heavy traffic
• How to make web sites load faster
Hosting Services
In the early days of the World Wide Web, individual organizations purchased their
own computer hardware, built their own computer rooms, acquired their own net-
work connections, and managed their own web server software.
As the Web quickly became mainstream, everyone wanted a web site, but few peo-
ple had the skills or time to build air-conditioned server rooms, register domain
412 |Chapter 18: Web Hosting
names, or purchase network bandwidth. To save the day, many new businesses
emerged, offering professionally managed web hosting services. Many levels of ser-
vice are available, from physical facilities management (providing space, air condi-
tioning, and wiring) to full-service web hosting, where all the customer does is
provide the content.
This chapter focuses on what the hosting web server provides. Much of what makes
a web site work—as well as, for example, its ability to support different languages
and its ability to do secure e-commerce transactions—depends on what capabilities
the hosting web server supports.
A Simple Example: Dedicated Hosting
Suppose that Joe’s Hardware Online and Mary’s Antique Auction both want fairly
high-volume web sites. Irene’s ISP has racks and racks full of identical, high-
performance web servers that it can lease to Joe and Mary, instead of having Joe and
Mary purchase their own servers and maintain the server software.
In Figure 18-1, both Joe and Mary sign up for the dedicated web hosting service
offered by Irene’s ISP. Joe leases a dedicated web server that is purchased and
maintained by Irene’s ISP. Mary gets a different dedicated server from Irene’s ISP.
Irene’s ISP gets to buy server hardware in volume and can select hardware that is
reliable, time-tested, and low-cost. If either Joe’s Hardware Online or Mary’s
Antique Auction grows in popularity, Irene’s ISP can offer Joe or Mary additional
servers immediately.
In this example, browsers send HTTP requests for www.joes-hardware.com to the IP
address of Joe’s server and requests for www.marys-antiques.com to the (different) IP
address of Mary’s server.
Figure 18-1. Outsourced dedicated hosting
Irene’s ISP
Internet
Client
Client
www.joes-hardware.com
www.cajun-gifts.com
www.marys-antiques.com
www.irenes-isp.com
Content
Joe
Content
Mary
Virtual Hosting |413
Virtual Hosting
Many folks want to have a web presence but don’t have high-traffic web sites. For
these people, providing a dedicated web server may be a waste, because they’re pay-
ing many hundreds of dollars a month to lease a server that is mostly idle!
Many web hosters offer lower-cost web hosting services by sharing one computer
between several customers. This is called shared hosting or virtual hosting. Each web
site appears to be hosted by a different server, but they really are hosted on the same
physical server. From the end user’s perspective, virtually hosted web sites should be
indistinguishable from sites hosted on separate dedicated servers.
For cost efficiency, space, and management reasons, a virtual hosting company
wants to host tens, hundreds, or thousands of web sites on the same server—but this
does not necessarily mean that 1,000 web sites are served from only one PC. Hosters
can create banks of replicated servers (called server farms) and spread the load across
the farm of servers. Because each server in the farm is a clone of the others, and hosts
many virtual web sites, administration is much easier. (We’ll talk more about server
farms in Chapter 20.)
When Joe and Mary started their businesses, they might have chosen virtual hosting
to save money until their traffic levels made a dedicated server worthwhile (see
Figure 18-2).
Virtual Server Request Lacks Host Information
Unfortunately, there is a design flaw in HTTP/1.0 that makes virtual hosters pull
their hair out. The HTTP/1.0 specification didn’t give any means for shared web
servers to identify which of the virtual web sites they’re hosting is being accessed.
Figure 18-2. Outsourced virtual hosting
Internet
Client
Client
Content
Joe
Content
Mary
Irene’s ISP
www.joes-hardware.com
www.cajun-gifts.com
www.marys-antiques.com
www.irenes-isp.com
414 |Chapter 18: Web Hosting
Recall that HTTP/1.0 requests send only the path component of the URL in the
request message. If you try to get http://www.joes-hardware.com/index.html, the
browser connects to the server www.joes-hardware.com, but the HTTP/1.0 request
says “GET /index.html”, with no further mention of the hostname. If the server is
virtually hosting multiple sites, this isn’t enough information to figure out what vir-
tual web site is being accessed. For example, in Figure 18-3:
• If client A tries to access http://www.joes-hardware.com/index.html, the request
“GET /index.html” will be sent to the shared web server.
• If client B tries to access http://www.marys-antiques.com/index.html, the identi-
cal request “GET /index.html” will be sent to the shared web server.
As far as the web server is concerned, there is not enough information to determine
which web site is being accessed! The two requests look the same, even though they
are for totally different documents (from different web sites). The problem is that the
web site host information has been stripped from the request.
As we saw in Chapter 6, HTTP surrogates (reverse proxies) and intercepting proxies
also need site-specifying information.
Making Virtual Hosting Work
The missing host information was an oversight in the original HTTP specification,
which mistakenly assumed that each web server would host exactly one web site.
HTTP’s designers didn’t provide support for virtually hosted, shared servers. For this
reason, the hostname information in the URL was viewed as redundant and stripped
away; only the path component was required to be sent.
Because the early specifications did not make provisions for virtual hosting, web
hosters needed to develop workarounds and conventions to support shared virtual
hosting. The problem could have been solved simply by requiring all HTTP request
Figure 18-3. HTTP/1.0 server requests don’t contain hostname information
Internet
Client B
Client A
(A getting http://www.joes-hardware.com/index.html)
GET /index.html HTTP/1.0
User-agent: SuperBrowser v1.3
GET /index.html HTTP/1.0
User-agent: WebSurfer 2000
(B getting http://www.marys-antiques.com/index.html)
/voting /mary /joe
www.voting-info.gov
www.joes-hardware.com
www.marys-antiques.com
HTTP/1.0 requests do not contain hostname information, so
they do not support web servers that host multiple web sites.
(HTTP/1.1 supports a Host header to fix this problem.)
Virtual Hosting |415
messages to send the full URL instead of just the path component. HTTP/1.1 does
require servers to handle full URLs in the request lines of HTTP messages, but it will
be a long time before all legacy applications are upgraded to this specification. In the
meantime, four techniques have emerged:
Virtual hosting by URL path
Adding a special path component to the URL so the server can determine the site.
Virtual hosting by port number
Assigning a different port number to each site, so requests are handled by sepa-
rate instances of the web server.
Virtual hosting by IP address
Dedicating different IP addresses for different virtual sites and binding all the IP
addresses to a single machine. This allows the web server to identify the site
name by IP address.
Virtual hosting by Host header
Many web hosters pressured the HTTP designers to solve this problem.
Enhanced versions of HTTP/1.0 and the official version of HTTP/1.1 define a
Host request header that carries the site name. The web server can identify the
virtual site from the Host header.
Let’s take a closer look at each technique.
Virtual hosting by URL path
You can use brute force to isolate virtual sites on a shared server by assigning them
different URL paths. For example, you could give each logical web site a special path
prefix:
• Joe’s Hardware store could be http://www.joes-hardware.com/joe/index.html.
• Mary’s Antiques store could be http://www.marys-antiques.com/mary/index.html.
When the requests arrive at the server, the hostname information is not present in
the request, but the server can tell them apart based on the path:
• The request for Joe’s hardware is “GET /joe/index.html”.
• The request for Mary’s antiques is “GET /mary/index.html”.
This is not a good solution. The “/joe” and “/mary” prefixes are redundant and con-
fusing (we already mentioned “joe” in the hostname). Worse, the common conven-
tion of specifying http://www.joes-hardware.com or http://www.joes-hardware.com/
index.html for the home page won’t work.
In general, URL-based virtual hosting is a poor solution and seldom is used.
Virtual hosting by port number
Instead of changing the pathname, Joe and Mary could each be assigned a different
port number on the web server. Instead of port 80, for example, Joe could get 82 and
416 |Chapter 18: Web Hosting
Mary could have 83. But this solution has the same problem: an end user would
expect to find the resources without having to specify a nonstandard port in the URL.
Virtual hosting by IP address
A much better approach (in common use) is virtual IP addressing. Here, each virtual
web site gets one or more unique IP addresses. The IP addresses for all of the virtual
web sites are attached to the same shared server. The server can look up the destina-
tion IP address of the HTTP connection and use that to determine what web site the
client thinks it is connected to.
Say a hoster assigned the IP address 209.172.34.3 to www.joes-hardware.com,
assigned 209.172.34.4 to www.marys-antiques.com, and tied both IP addresses to the
same physical server machine. The web server could then use the destination IP
address to identify which virtual site is being requested, as shown in Figure 18-4:
• Client A fetches http://www.joes-hardware.com/index.html.
• Client A finds the IP address for www.joes-hardware.com, getting 209.172.34.3.
• Client A opens a TCP connection to the shared web server at 209.172.34.3.
• Client A sends the request “GET /index.html HTTP/1.0”.
• Before the web server serves a response, it notes the actual destination IP address
(209.172.34.3), determines that this is a virtual IP address for Joe’s web site, and
fulfills the request from the /joe subdirectory. The page /joe/index.html is returned.
Similarly, if client B asks for http://www.marys-antiques.com/index.html:
• Client B finds the IP address for www.marys-antiques.com, getting 209.172.34.4.
• Client B opens a TCP connection to the web server at 209.172.34.4.
• Client B sends the request “GET /index.html HTTP/1.0”.
• The web server determines that 209.172.34.4 is Mary’s web site and fulfills the
request from the /mary subdirectory, returning the document /mary/index.html.
Figure 18-4. Virtual IP hosting
Dest IP address
209.172.34.2
209.172.34.3
209.172.34.4
Directory
/voting
/joe
/mary
Internet
Client B
Client A
www.voting-info.gov= 209.172.34.2
www.joes-hardware.com= 209.172.34.3
www marys-antiques.com= 209.172.34.4
/voting /mary /joe
209.172.34.3
209.172.34.4
Virtual Hosting |417
Virtual IP hosting works, but it causes some difficulties, especially for large hosters:
• Computer systems usually have a limit on how many virtual IP addresses can be
bound to a machine. Hosters that want hundreds or thousands of virtual sites to
be hosted on a shared server may be out of luck.
• IP addresses are a scarce commodity. Hosters with many virtual sites might not
be able to obtain enough virtual IP addresses for the hosted web sites.
• The IP address shortage is made worse when hosters replicate their servers for
additional capacity. Different virtual IP addresses may be needed on each repli-
cated server, depending on the load-balancing architecture, so the number of IP
addresses needed can multiply by the number of replicated servers.
Despite the address consumption problems with virtual IP hosting, it is used widely.
Virtual hosting by Host header
To avoid excessive address consumption and virtual IP limits, we’d like to share the
same IP address among virtual sites, but still be able to tell the sites apart. But as
we’ve seen, because most browsers send just the path component of the URL to serv-
ers, the critical virtual hostname information is lost.
To solve this problem, browser and server implementors extended HTTP to provide
the original hostname to servers. But browsers couldn’t just send a full URL,
because that would break many servers that expected to receive only a path compo-
nent. Instead, the hostname (and port) is passed in a Host extension header in all
requests.
In Figure 18-5, client A and client B both send Host headers that carry the original
hostname being accessed. When the server gets the request for /index.html, it can use
the Host header to decide which resources to use.
Figure 18-5. Host headers distinguish virtual host requests
Internet
Client B
Client A
(A getting http://www.joes-hardware.com/index.html)
GET /index.html HTTP/1.1
User-agent: SuperBrowser v1.3
Host: www.joes-hardware.com
GET /index.html HTTP/1.1
User-agent: WebSurfer 2000
Host: marys-antiques.com
(B getting http://www.marys-antiques.com/index.html)
/voting /mary /joe
www.voting-info.gov
www.joes-hardware.com
www.marys-antiques.com
The HTTP Host header carries the hostname information that would
otherwise be lost in normal server requests, allowing name-based
virtual hosting.
418 |Chapter 18: Web Hosting
Host headers were first introduced with HTTP/1.0+, a vendor-extended superset of
HTTP/1.0. Host headers are required for HTTP/1.1 compliance. Host headers are
supported by most modern browsers and servers, but there are still a few clients and
servers (and robots) that don’t support them.
HTTP/1.1 Host Headers
The Host header is an HTTP/1.1 request header, defined in RFC 2068. Virtual serv-
ers are so common that most HTTP clients, even if they are not HTTP/1.1-compliant,
implement the Host header.
Syntax and usage
The Host header specifies the Internet host and port number for the resource being
requested, as obtained from the original URL:
Host = "Host" ":" host [ ":" port ]
In particular:
• If the Host header does not contain a port, the default port for the scheme is
assumed.
• If the URL contains an IP address, the Host header should contain the same
address.
• If the URL contains a hostname, the Host header must contain the same name.
• If the URL contains a hostname, the Host header should not contain the IP
address equivalent to the URL’s hostname, because this will break virtually
hosted servers, which layer multiple virtual sites over a single IP address.
• If the URL contains a hostname, the Host header should not contain another
alias for this hostname, because this also will break virtually hosted servers.
• If the client is using an explicit proxy server, the client must include the name
and port of the origin server in the Host header, not the proxy server. In the past,
several web clients had bugs where the outgoing Host header was set to the host-
name of the proxy, when the client’s proxy setting was enabled. This incorrect
behavior causes proxies and origin servers to misbehave.
• Web clients must include a Host header field in all request messages.
• Web proxies must add Host headers to request messages before forwarding them.
• HTTP/1.1 web servers must respond with a 400 status code to any HTTP/1.1
request message that lacks a Host header field.
Here is a sample HTTP request message used to fetch the home page of www.joes-
hardware.com, along with the required Host header field:
GET http://www.joes-hardware.com/index.html HTTP/1.0
Connection: Keep-Alive
User-Agent: Mozilla/4.51 [en] (X11; U; IRIX 6.2 IP22)
Making Web Sites Reliable |419
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*
Accept-Encoding: gzip
Accept-Language: en
Host: www.joes-hardware.com
Missing Host headers
A small percentage of old browsers in use do not send Host headers. If a virtual host-
ing server is using Host headers to determine which web site to serve, and no Host
header is present, it probably will either direct the user to a default web page (such as
the web page of the ISP) or return an error page suggesting that the user upgrade her
browser.
Interpreting Host headers
An origin server that isn’t virtually hosted, and doesn’t allow resources to differ by
the requested host, may ignore the Host header field value. But any origin server that
does differentiate resources based on the host must use the following rules for deter-
mining the requested resource on an HTTP/1.1 request:
1. If the URL in the HTTP request message is absolute (i.e., contains a scheme and
host component), the value in the Host header is ignored in favor of the URL.
2. If the URL in the HTTP request message doesn’t have a host, and the request con-
tains a Host header, the value of the host/port is obtained from the Host header.
3. If no valid host can be determined through Steps 1 or 2, a 400 Bad Response
response is returned to the client.
Host headers and proxies
Some browser versions send incorrect Host headers, especially when configured to
use proxies. For example, when configured to use a proxy, some older versions of
Apple and PointCast clients mistakenly sent the name of the proxy instead of the ori-
gin server in the Host header.
Making Web Sites Reliable
There are several times during which web sites commonly break:
• Server downtime
• Traffic spikes: suddenly everyone wants to see a particular news broadcast or
rush to a sale. Sudden spikes can overload a web server, slowing it down or stop-
ping it completely.
• Network outages or losses
This section presents some ways of anticipating and dealing with these common
problems.
420 |Chapter 18: Web Hosting
Mirrored Server Farms
A server farm is a bank of identically configured web servers that can cover for each
other. The content on each server in the farm can be mirrored, so that if one has a
problem, another can fill in.
Often, mirrored servers follow a hierarchical relationship. One server might act as
the “content authority”—the server that contains the original content (perhaps a
server to which the content authors post). This server is called the master origin
server. The mirrored servers that receive content from the master origin server are
called replica origin servers. One simple way to deploy a server farm is to use a net-
work switch to distribute requests to the servers. The IP address for each of the web
sites hosted on the servers is the IP address of the switch.
In the mirrored server farm shown in Figure 18-6, the master origin server is respon-
sible for sending content to the replica origin servers. To the outside world, the IP
address for this content is the IP address of the switch. The switch is responsible for
sending requests to the servers.
Mirrored web servers can contain copies of the exact same content at different loca-
tions. Figure 18-7 illustrates four mirrored servers, with a master server in Chicago
and replicas in New York, Miami, and Little Rock. The master server serves clients
in the Chicago area and also has the job of propagating its content to the replica
servers.
In the Figure 18-7 scenario, there are a couple of ways that client requests would be
directed to a particular server:
HTTP redirection
The URL for the content could resolve to the IP address of the master server,
which could then send redirects to replica servers.
Figure 18-6. Mirrored server farm
Internet
Replica origin server
Master origin server
Replica origin servers
Client
Client
Client
Client Client
Switch
Making Web Sites Reliable |421
DNS redirection
The URL for the content could resolve to four IP addresses, and the DNS server
could choose the IP address that it sends to clients.
See Chapter 20 for more details.
Content Distribution Networks
Acontent distribution network (CDN) is simply a network whose purpose is the dis-
tribution of specific content. The nodes of the network can be web servers, surro-
gates, or proxy caches.
Surrogate Caches in CDNs
Surrogate caches can be used in place of replica origin servers in Figures 18-6 and
18-7. Surrogates, also known as reverse proxies, receive server requests for content
just as mirrored web servers do. They receive server requests on behalf of a specific
set of origin servers (this is possible because of the way IP addresses for content are
advertised; there usually is a working relationship between origin server and surro-
gate, and surrogates expect to receive requests aimed at specific origin servers).
The difference between a surrogate and a mirrored server is that surrogates typically
are demand-driven. They do not store entire copies of the origin server content; they
store whatever content their clients request. The way content is distributed in their
caches depends on the requests that they receive; the origin server does not have the
responsibility to update their content. For easy access to “hot” content (content that
is in high demand), some surrogates have “prefetching” features that enable them to
pull content in advance of user requests.
An added complexity in CDNs with surrogates is the possibility of cache hierarchies.
Figure 18-7. Dispersed mirrored servers
Internet
Chicago (HQ)
Master origin server
Little Rock
Replica origin server
New York
Replica origin server
Miami
Replica origin server
422 |Chapter 18: Web Hosting
Proxy Caches in CDNs
Proxy caches also can be deployed in configurations similar to those in Figures 18-6
and 18-7. Unlike surrogates, traditional proxy caches can receive requests aimed at
any web servers (there need not be any working relationship or IP address agree-
ment between a proxy cache and an origin server). As with surrogates, however,
proxy cache content typically is demand-driven and is not expected to be an exact
duplicate of the origin server content. Some proxy caches also can be preloaded with
hot content.
Demand-driven proxy caches can be deployed in other kinds of configurations—in
particular, interception configurations, where a layer-2 or -3 device (switch or router)
intercepts web traffic and sends it to a proxy cache (see Figure 18-8).
An interception configuration depends on being able to set up the network between
clients and servers so that all of the appropriate HTTP requests are physically chan-
neled to the cache. (See Chapter 20). The content is distributed in the cache accord-
ing to the requests it receives.
Making Web Sites Fast
Many of the technologies mentioned in the previous section also help web sites load
faster. Server farms and distributed proxy caches or surrogate servers distribute net-
work traffic, avoiding congestion. Distributing the content brings it closer to end
users, so that the travel time from server to client is lower. The key to speed of
resource access is how requests and responses are directed from client to server and
back across the Internet. See Chapter 20 for details on redirection methods.
Another approach to speeding up web sites is encoding the content for fast transpor-
tation. This can mean, for example, compressing the content, assuming that the
receiving client can uncompress it. See Chapter 15 for details.
Figure 18-8. Client requests intercepted by a switch and sent to a proxy
Internet and lots
of origin servers
Client
Client
Client
Client Caching proxy
Switch
For More Information |423
For More Information
See Part III, Identification, Authorization, and Security, for details on how to make
web sites secure. The following Internet drafts and documentation can give you more
details about web hosting and content distribution:
http://www.ietf.org/rfc/rfc3040.txt
RFC 3040, “Internet Web Replication and Caching Taxonomy,” is a reference
for the vocabulary of web replication and caching applications.
http://search.ietf.org/internet-drafts/draft-ietf-cdi-request-routing-reqs-00.txt
“Request-Routing Requirements for Content Internetworking.”
Apache: The Definitive Guide
Ben Laurie and Peter Laurie, O’Reilly & Associates, Inc. This book describes
how to run the open source Apache web server.
424
CHAPTER 19
Publishing Systems
How do you create web pages and get them onto a web server? In the dark ages of
the Web (let’s say, 1995), you might have hand-crafted your HTML in a text editor
and manually uploaded the content to the web server using FTP. This procedure was
painful, difficult to coordinate with coworkers, and not particularly secure.
Modern-day publishing tools make it much more convenient to create, publish, and
manage web content. Today, you can interactively edit web content as you’ll see it
on the screen and publish that content to servers with a single click, while being noti-
fied of any files that have changed.
Many of the tools that support remote publishing of content use extensions to the
HTTP protocol. In this chapter, we explain two important technologies for web-
content publishing based on HTTP: FrontPage and DAV.
FrontPage Server Extensions
for Publishing Support
FrontPage (commonly referred to as FP) is a versatile web authoring and publishing
toolkit provided by Microsoft Corp. The original idea for FrontPage (FrontPage 1.0)
was conceived in 1994, at Vermeer Technologies, Inc., and was dubbed the first
product to combine web site management and creation into a single, unified tool.
Microsoft purchased Vermeer and shipped FrontPage 1.1 in 1996. The latest ver-
sion, FrontPage Version 2002, is the sixth version in the line and a core part of the
Microsoft Office suite.
FrontPage Server Extensions
As part of the “publish anywhere” strategy, Microsoft released a set of server-side soft-
ware called FrontPage Server Extensions (FPSE). These server-side components inte-
grate with the web server and provide the necessary translation between the web site
and the client running FrontPage (and other clients that support these extensions).
FrontPage Server Extensions for Publishing Support |425
Our primary interest lies in the publishing protocol between the FP clients and FPSE.
This protocol provides an example of designing extensions to the core services avail-
able in HTTP without changing HTTP semantics.
The FrontPage publishing protocol implements an RPC layer on top of the HTTP
POST request. This allows the FrontPage client to send commands to the server to
update documents on the web site, perform searches, collaborate amongst the web
authors, etc. Figure 19-1 gives an overview of the communication.
The web server sees POST requests addressed to the FPSE (implemented as a set of
CGI programs, in the case of a non-Microsoft IIS server) and directs those requests
accordingly. As long as intervening firewalls and proxy servers are configured to
allow the POST method, FrontPage can continue communicating with the server.
FrontPage Vocabulary
Before we dive deeper into the RPC layer defined by FPSE, it may help to establish
the common vocabulary:
Virtual server
One of the multiple web sites running on the same server, each with a unique
domain name and IP address. In essence, a virtual server allows a single web
server to host multiple web sites, each of which appears to a browser as being
hosted by its own web server. A web server that supports virtual servers is called
amulti-hosting web server. A machine that is configured with multiple IP
addresses is called a multi-homed server (for more details, please refer to “Vir-
tual Hosting” in Chapter 18).
Root web
The default, top-level content directory of a web server, or, in a multi-hosting
environment, the top-level content directory of a virtual web server. To access
the root web, it is enough to specify the URL of the server without specifying a
page name. There can be only one root web per web server.
Figure 19-1. FrontPage publishing architecture
Internet
HTTP request message contains
the command and the URL www.joes-hardware.com
FrontPage clients:
FrontPage,
MS Word, Excel, etc.
HTTP
CGI ISAPI
FrontPage Server
Extensions (FPSE)
HTTP
426 |Chapter 19: Publishing Systems
Subweb
A named subdirectory of the root web or another subweb that is a complete
FPSE extended web. A subweb can be a complete independent entity with the
ability to specify its own administration and authoring permissions. In addition,
subwebs may provide scoping for methods such as searches.
The FrontPage RPC Protocol
The FrontPage client and FPSE communicate using a proprietary RPC protocol. This
protocol is layered on top of HTTP POST by embedding the RPC methods and their
associated variables in the body of the POST request.
To start the process, the client needs to determine the location and the name of the
target programs on the server (the part of the FPSE package that can execute the
POST request). It then issues a special GET request (see Figure 19-2).
When the file is returned, the FrontPage client reads the response and finds the val-
ues associated with FPShtmlScriptUrl,FPAuthorScriptUrl, and FPAdminScriptUrl.
Typically, this may look like:
FPShtmlScriptUrl="_vti_bin/_vti_rpc/shtml.dll"
FPAuthorScriptUrl="_vti_bin/_vti_aut/author.dll"
FPAdminScriptUrl="_vti_bin/_vti_adm/admin.dll"
FPShtmlScriptUrl tells the client where to POST requests for “browse time” com-
mands (e.g., getting the version of FPSE) to be executed.
FPAuthorScriptUrl tells the client where to POST requests for “authoring time” com-
mands to be executed. Similarly, FPAdminScriptUrl tells FrontPage where to POST
requests for administrative actions.
Figure 19-2. Initial request
Internet
HTTP request message contains
the command and the URL
FrontPage clients
GET /_vti_inf.html HTTP/1.1
Date: Sat, 12 Aug 2000 20:31:24 GMT
User-agent: Mozilla/2.0 (compatible;MS FrontPage 4.0)
Host: taskserver:80
Accept: auth/sicily
Content-length: 0 www.joes-hardware.com
CGI ISAPI
FrontPage Server
Extensions (FPSE)
FrontPage Server Extensions for Publishing Support |427
Now that we know where the various programs are located, we are ready to send a
request.
Request
The body of the POST request contains the RPC command, in the form of
“method=<command>” and any required parameters. For example, consider the
RPC message requesting a list of documents, as follows:
POST /_vti_bin/_vti_aut/author.dll HTTP/1.1
Date: Sat, 12 Aug 2000 20:32:54 GMT
User-Agent: MSFrontPage/4.0
..........................................
<BODY>
method=list+documents%3a4%2e0%2e2%2e3717&service%5fname=&listHiddenDocs=false&listExp
lorerDocs=false&listRecurse=false&listFiles=true&listFolders=true&listLinkInfo=true&l
istIncludeParent=true&listDerived=false
&listBorders=false&listChildWebs=true&initialUrl=&folderList=%5b%3bTW%7c12+Aug+2000+2
0%3a33%3a04+%2d0000%5d
The body of the POST command contains the RPC command being sent to the
FPSE. As with CGI programs, the spaces in the method are encoded as plus sign (+)
characters. All other nonalphanumeric characters in the method are encoded using
%XX format, where the XX stands for the ASCII representation of the character. Using
this notation, a more readable version of the body would look like the following:
method=list+documents:4.0.1.3717
&service_name=
&listHiddenDocs=false
&listExplorerDocs=false
.....
Some of the elements listed are:
service_name
The URL of the web site on which the method should act. Must be an existing
folder or one level below an existing folder.
listHiddenDocs
Shows the hidden documents in a web if its value is “true”. The “hidden” docu-
ments are designated by URLs with path components starting with “_”.
listExploreDocs
If the value is “true”, lists the task lists.
Response
Most RPC protocol methods have return values. Most common return values are for
successful methods and errors. Some methods also have a third subsection, “Sample
Return Code.” FrontPage properly interprets the codes to provide accurate feedback
to the user.
428 |Chapter 19: Publishing Systems
Continuing with our example, the FPSE processes the “list+documents” request and
returns the necessary information. A sample response follows:
HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Sat, 12 Aug 2000 22:49:50 GMT
Content-type: application/x-vermeer-rpc
X-FrontPage-User-Name: IUSER_MINSTAR
<html><head><title>RPC packet</title></head>
<body>
<p>method=list documents: 4.0.2.3717
<p>document_list=
<ul>
<li>document_name=help.gif
<\ul>
As you can see from the response, a formatted list of documents available on the web
server is returned to the FP client. You can find the complete list of commands and
responses at the Microsoft web site.
FrontPage Security Model
Any publishing system directly accessing web server content needs to be very con-
scious of the security implications of its actions. For the most part, FPSE depends on
the web server to provide the security.
The FPSE security model defines three kinds of users: administrators, authors, and
browsers, with administrators having complete control. All permissions are cumula-
tive; i.e., all administrators may author and browse the FrontPage web. Similarly, all
authors have browsing permissions.
The list of administrators, authors, and browsers is defined for a given FPSE
extended web. All of the subwebs may inherit the permissions from the root web or
set their own. For non-IIS web servers, all the FPSE programs are required to be
stored in directories marked “executable” (the same restriction as for any other CGI
program). Fpsrvadm, the FrontPage server administrator utility, may be used for this
purpose. On IIS servers, the integrated Windows security model prevails.
On non-IIS servers, web server access-control mechanisms specify the users who are
allowed to access a given program. On Apache and NCSA web servers, the file is
named .htaccess; on Netscape servers, it is named .nsconfig. The access file associ-
ates users, groups, and IP addresses with various levels of permissions: GET (read),
POST (execute), etc. For example, for a user to be an author on an Apache web
server, the .htaccess file should permit that user to POST to author.exe. These access-
specification files often are defined on a per-directory basis, providing greater flexi-
bility in defining the permissions.
WebDAV and Collaborative Authoring |429
On IIS servers, the permissions are checked against the ACLs for a given root or sub-
root. When IIS gets a request, it first logs on and impersonates the user, then sends
the request to one of the three extension dynamic link libraries (DLLs). The DLL
checks the impersonation credentials against the ACL defined for the destination
folder. If the check is successful, the requested operation is executed by the exten-
sion DLL. Otherwise, a “permission denied” message is sent back to the client.
Given the tight integration of Windows security with IIS, the User Manager may be
used to define fine-grained control.
In spite of this elaborate security model, enabling FPSE has gained notoriety as a
nontrivial security risk. In most cases, this is due to sloppy practices adopted by web
site administrators. However, the earlier versions of FPSE did have severe security
loopholes and thus contributed to the general perception of security risk. This prob-
lem also was exacerbated by the arcane practices needed to fully implement a tight
security model.
WebDAV and Collaborative Authoring
Web Distributed Authoring and Versioning (WebDAV) adds an extra dimension to
web publishing—collaboration. Currently, the most common practice of collabora-
tion is decidedly low-tech: predominantly email, sometimes combined with distrib-
uted fileshares. This practice has proven to be very inconvenient and error-prone,
with little or no control over the process. Consider an example of launching a multi-
national, multilingual web site for an automobile manufacturer. It’s easy to see the
need for a robust system with secure, reliable publishing primitives, along with col-
laboration primitives such as locking and versioning.
WebDAV (published as RFC 2518) is focused on extending HTTP to provide a suit-
able platform for collaborative authoring. It currently is an IETF effort with support
from various vendors, including Adobe, Apple, IBM, Microsoft, Netscape, Novell,
Oracle, and Xerox.
WebDAV Methods
WebDAV defines a set of new HTTP methods and modifies the operational scope of
a few other HTTP methods. The new methods added by WebDAV are:
PROPFIND
Retrieves the properties of a resource.
PROPPATCH
Sets one or more properties on one or many resources.
MKCOL
Creates collections.
430 |Chapter 19: Publishing Systems
COPY
Copies a resource or a collection of resources from a given source to a given des-
tination. The destination need not be on the same machine.
MOVE
Moves a resource or a collection of resources from a given source to a given des-
tination. The destination need not be on the same machine.
LOCK
Locks a resource or multiple resources.
UNLOCK
Unlocks a previously locked resource.
HTTP methods modified by WebDAV are DELETE, PUT, and OPTIONS. Both the
new and the modified methods are discussed in detail later in this chapter.
WebDAV and XML
WebDAV’s methods generally require a great deal of information to be associated
with both requests and responses. HTTP usually communicates this information in
message headers. However, transporting necessary information in headers alone
imposes some limitations, including the difficulties of selective application of header
information to multiple resources in a request, to represent hierarchy, etc.
To solve this problem, WebDAV embraces the Extensible Markup Language (XML),
a meta-markup language that provides a format for describing structured data. XML
provides WebDAV with:
• A method of formatting instructions describing how data is to be handled
• A method of formatting complex responses from the server
• A method of communicating customized information about the collections and
resources handled
• A flexible vehicle for the data itself
• A robust solution for most of the internationalization issues
Traditionally, the schema definition for XML documents is kept in a Document Type
Definition (DTD) file that is referenced within the XML document itself. Therefore,
when trying to interpret an XML document, the DOCTYPE definition entity gives
the name of the DTD file associated with the XML document in question.
WebDAV defines an explicit XML namespace, “DAV:”. Without going into many
details, an XML namespace is a collection of names of elements or attributes. The
namespace qualifies the embedded names uniquely across the domain, thus avoid-
ing any name collisions.
The complete XML schema is defined in the WebDAV specification, RFC 2518. The
presence of a predefined schema allows the parsing software to make assumptions on
the XML schema without having to read in DTD files and interpret them correctly.
WebDAV and Collaborative Authoring |431
WebDAV Headers
WebDAV does introduce several HTTP headers to augment the functionality of the
new methods. This section provides a brief overview; see RFC 2518 for more infor-
mation. The new headers are:
DAV
Used to communicate the WebDAV capabilities of the server. All resources sup-
ported by WebDAV are required to return this header in the response to the
OPTIONS request. See “The OPTIONS method” for more details.
DAV = "DAV" ":" "1" ["," "2"] ["," 1#extend]
Depth
The crucial element for extending WebDAV to grouped resources with multiple
levels of hierarchy (for more detailed explanation about collections, please refer
to “Collections and Namespace Management”).
Depth = "Depth" ":" ("0" | "1" | "infinity")
Let’s look at a simple example. Consider a directory DIR_A with files file_1.html
and file_2.html. If a method uses Depth: 0, the method applies to the DIR_A direc-
tory alone, and Depth: 1 applies to the DIR_A directory and its files, file_1.html
and file_2.html.
The Depth header modifies many WebDAV-defined methods. Some of the
methods that use the Depth header are LOCK, COPY, and MOVE.
Destination
Defined to assist the COPY or MOVE methods in identifying the destination
URI.
Destination = "Destination" ":" absoluteURI
If
The only defined state token is a lock token (see “The LOCK Method”). The If
header defines a set of conditionals; if they all evaluate to false, the request will
fail. Methods such as COPY and PUT conditionalize the applicability by specify-
ing preconditions in the If header. In practice, the most common precondition to
be satisfied is the prior acquisition of a lock.
If = "If" ":" (1*No-tag-list | 1*Tagged-list)
No-tag-list = List
Tagged-list = Resource 1*List
Resource = Coded-URL
List = "(" 1*(["Not"](State-token | "[" entity-tag "]")) ")"
State-token = Coded-URL
Coded-URL = "<" absoluteURI ">"
Lock-Token
Used by the UNLOCK method to specify the lock that needs to be removed. A
response to a LOCK method also has a Lock-Token header, carrying the neces-
sary information about the lock taken.
Lock-Token = "Lock-Token" ":" Coded-URL
432 |Chapter 19: Publishing Systems
Overwrite
Used by the COPY and MOVE methods to designate whether the destination
should be overwritten. See the discussion of the COPY and MOVE methods
later in this chapter for more details.
Overwrite = "Overwrite" ":" ("T" | "F")
Timeout
A request header used by a client to specify a desired lock timeout value. For
more information, refer to the section “Lock refreshes and the Timeout header.”
TimeOut = "Timeout" ":" 1#TimeType
TimeType = ("Second-" DAVTimeOutVal | "Infinite" | Other)
DAVTimeOutVal = 1*digit
Other = "Extend" field-value
Now that we have sketched the intent and implementation of WebDAV, let’s look
more closely at the functions provided.
WebDAV Locking and Overwrite Prevention
By definition, collaboration requires more than one person working on a given docu-
ment. The inherent problem associated with collaboration is illustrated in Figure 19-3.
In this example, authors A and B are jointly writing a specification. A and B indepen-
dently make a set of changes to the document. A pushes the updated document to
the repository, and at a later point, B posts her own version of the document into the
repository. Unfortunately, because B never knew about A’s changes, she never
merged her version with A’s version, resulting in A’s work being lost.
Figure 19-3. Lost update problem
Author A Shared file repository
A’s copy
Author B
B’s copy
Author A Shared file repository
“A” publishes. . .
Author B
B’s copy
Author A Shared file repository Author B
“B” also publishes and
overwrites “A”’s changes
WebDAV and Collaborative Authoring |433
To ameliorate the problem, WebDAV supports the concept of locking. Locking
alone will not fully solve the problem. Versioning and messaging support are needed
to complete the solution.
WebDAV supports two types of locks:
• Exclusive write locking of a resource or a collection
• Shared write locking of a resource or a collection
An exclusive write lock guarantees write privileges only to the lock owner. This type
of locking completely eliminates potential conflicts. A shared write lock allows a
group of people to work on a given document. This type of locking works well in an
environment where all the authors are aware of each other’s activities. WebDAV pro-
vides a property discovery mechanism, via PROPFIND, to determine the support for
locking and the types of locks supported.
WebDAV has two new methods to support locking: LOCK and UNLOCK.
To accomplish locking, there needs to be a mechanism for identifying the author.
WebDAV requires digest authentication (discussed in Chapter 13).
When a lock is granted, the server returns a token that is unique across the domain
to the client. The specification refers to this as the opaquelocktoken lock token URI
scheme. When the client subsequently wants to perform a write, it connects to the
server and completes the digest authentication sequence. Once the authentication is
complete, the WebDAV client presents the lock token, along with the PUT request.
Thus, the combination of the correct user and the lock token is required to complete
the write.
The LOCK Method
A powerful feature of WebDAV is its ability to lock multiple resources with a single
LOCK request. WebDAV locking does not require the client to stay connected to the
server.
For example, here’s a simple LOCK request:
LOCK /ch-publish.fm HTTP/1.1
Host: minstar
Content-Type: text/xml
User-Agent: Mozilla/4.0 (compatible; MSIE 5.0; Windows NT)
Content-Length: 201
<?xml version="1.0"?>
<a:lockinfo xmlns:a="DAV:">
<a:lockscope><a:exclusive/></a:lockscope>
<a:locktype><a:write/></a:locktype>
<a:owner><a:href>AuthorA</a:href></a:owner>
</a:lockinfo>
434 |Chapter 19: Publishing Systems
The XML being submitted has the <lockinfo> element as its base element. Within
the <lockinfo> structure, there are three subelements:
<locktype>
Indicates the type of lock. Currently there is only one, “write.”
<lockscope>
Indicates whether this is an exclusive lock or a shared lock.
<owner>
Field is set with the person who holds the current lock.
Here’s a successful response to our LOCK request:
HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Fri, 10 May 2002 20:56:18 GMT
Content-Type: text/xml
Content-Length: 419
<?xml version="1.0"?>
<a:prop xmlns:a="DAV:">
<a:lockdiscovery><a:activelock>
<a:locktype><a:write/></a:locktype>
<a:lockscope><a:exclusive/></a:lockscope>
<a:owner xmlns:a="DAV:"><a:href>AutherA</a:href></a:owner>
<a:locktoken><a:href>opaquelocktoken:*****</a:href></a:locktoken>
<a:depth>0</a:depth>
<a:timeout>Second-180</a:timeout>
</a:activelock></a:lockdiscovery>
</a:prop>
The <lockdiscovery> element acts as a container for information about the lock.
Embedded in the <lockdiscovery> element is an <activelock> subelement that holds
the information sent with the request (<locktype>, <lockscope>, and <owner>). In
addition, <activelock> has the following subelements:
<locktoken>
Uniquely identifies the lock in a URI scheme called opaquelocktoken. Given the
stateless nature of HTTP, this token is used to identify the ownership of the lock
in future requests.
<depth>
Mirrors the value of the Depth header.
<timeout>
Indicates the timeout associated with the lock. In the above response
(Figure 19-3), the timeout value is 180 seconds.
The opaquelocktoken scheme
The opaquelocktoken scheme is designed to provide a unique token across all resources
for all times. To guarantee uniqueness, the WebDAV specification mandates the use of
the universal unique identifier (UUID) mechanism, as described in ISO-11578.
WebDAV and Collaborative Authoring |435
When it comes to actual implementation, there is some leeway. The server has the
choice of generating a UUID for each LOCK request, or generating a single UUID
and maintaining the uniqueness by appending extra characters at the end. For per-
formance considerations, the latter choice is better. However, if the server chooses to
implement the latter choice, it is required to guarantee that none of the added exten-
sions will ever be reused.
The <lockdiscovery> XML element
The <lockdiscovery> XML element provides a mechanism for active lock discovery.
If others try to lock the file while a lock is in place, they will receive a <lockdiscov-
ery> XML element that indicates the current owner. The <lockdiscovery> element
lists all outstanding locks along with their properties.
Lock refreshes and the Timeout header
To refresh a lock, a client needs to resubmit a lock request with the lock token in the
If header. The timeout value returned may be different from the earlier timeout values.
Instead of accepting the timeout value given by the server, a client may indicate the
timeout value required in the LOCK request. This is done through the Timeout
header. The syntax of the Timeout header allows the client to specify a few options
in a comma-separated list. For example:
Timeout : Infinite, Second-86400
The server is not obligated to honor either of the options. However, it is required to
provide the lock expiration time in the <timeout> XML element. In all cases, lock
timeout is only a guideline and is not necessarily binding on the server. The adminis-
trator may do a manual reset, or some other extraordinary event may cause the
server to reset the lock. The clients should avoid taking lengthy locks.
In spite of these primitives, we may not completely solve the “lost update problem”
illustrated in Figure 19-3. To completely solve it, a cooperative event system with a
versioning control is needed.
The UNLOCK Method
The UNLOCK method removes a lock on a resource, as follows:
UNLOCK /ch-publish.fm HTTP/1.1
Host: minstar.inktomi.com
User-Agent: Mozilla/4.0 (compatible; MSIE 5.0; Windows NT)
Lock-Token:
opaquelocktoken:*********
HTTP/1.1 204 OK
Server: Microsoft-IIS/5.0
Date: Fri, 10 May 2002 20:56:18 GMT
436 |Chapter 19: Publishing Systems
As with most resource management requests, WebDAV has two requirements for
UNLOCK to succeed: prior completion of a successful digest authentication
sequence, and matching the lock token that is sent in the Lock-Token header.
If the unlock is successful, a 204 No Content status code is returned to client.
Table 19-1 summarizes the possible status codes with the LOCK and UNLOCK
methods.
Properties and META Data
Properties describe information about the resource, including the author’s name,
modification date, content rating, etc. META tags in HTML do provide a mecha-
nism to embed this information as part of the content; however, many resources
(such as any binary data) have no capability for embedding META data.
A distributed collaborative system such as WebDAV adds more complexity to the
property requirement. For example, consider an author property: when a document
gets edited, this property needs to be updated to reflect the new authors. WebDAV
terms such dynamically modifiable properties “live” properties. The more perma-
nent, static properties, such as Content-Type, are termed “dead” properties.
To support discovery and modification of properties, WebDAV extends HTTP to
include two new methods, PROPFIND and PROPPATCH. Examples and corre-
sponding XML elements are described in the following sections.
Table 19-1. Status codes for LOCK and UNLOCK methods
Status code Defined by Method Effect
200 OK HTTP LOCK Indicates successful locking.
201 Created HTTP LOCK Indicates that a lock on a nonexistent resource succeeded by cre-
ating the resource.
204 No Content HTTP UNLOCK Indicates successful unlocking.
207 Multi-Status WebDAV LOCK The request was for locking multiple resources. Not all status
codes returned were the same. Hence, they are all encapsulated
in a 207 response.
403 Forbidden HTTP LOCK Indicates that the client does not have permission to lock the
resource.
412 Precondition Failed HTTP LOCK Either the XML sent with the LOCK command indicated a condi-
tion to be satisfied and the server failed to complete the required
condition, or the lock token could not be enforced.
422 Unprocessable Property WebDAV LOCK Inapplicable semantics—an example may be specifying a non-
zero Depth for a resource that is not a collection.
423 Locked WebDAV LOCK Already locked.
424 Failed Dependency WebDAV UNLOCK UNLOCK specifies other actions and their success as a condition
for the unlocking. This error is returned if the dependency fails to
complete.
WebDAV and Collaborative Authoring |437
The PROPFIND Method
The PROPFIND (property find) method is used for retrieving the properties of a
given file or a group of files (also known as a “collection”). PROPFIND supports
three types of operations:
• Request all properties and their values.
• Request a selected set of properties and values.
• Request all property names.
Here’s the scenario where all the properties and their values are requested:
PROPFIND /ch-publish.fm HTTP/1.1
Host: minstar.inktomi.com
User-Agent: Mozilla/4.0 (compatible; MSIE 5.0; Windows NT)
Depth: 0
Cache-Control: no-cache
Connection: Keep-Alive
Content-Length: 0
The <propfind> request element specifies the properties to be returned from a
PROPFIND method. The following list summarizes a few XML elements that are
used with PROPFIND requests:
<allprop>
Requires all property names and values to be returned. To request all properties
and their values, a WebDAV client may either send an <allprop> XML subele-
ment as part of the <propfind> element, or submit a request with no body.
<propname>
Specifies the set of property names to be returned.
<prop>
A subelement of the <propfind> element. Specifies a specific property whose
value is to be returned. For example: “<a:prop> <a:owner />..... </a:prop>”.
Here’s a response to a sample PROPFIND request:
HTTP/1.1 207 Multi-Status
Server: Microsoft-IIS/5.0
...........
<?xml version="1.0"?>
<a:multistatusxmlns:b="urn:uuid:********/" xmlns:c="xml:" xmlns:a="DAV:">
<a:response>
<a:href>http://minstar/ch-publish.fm </a:href>
<a:propstat>
<a:status>HTTP/1.1 200OK</a:status>
<a:prop>
<a:getcontentlength b:dt="int">1155</a:getcontentlength>
......................
......................
438 |Chapter 19: Publishing Systems
<a:ishidden b:dt="boolean">0</a:ishidden>
<a:iscollection b:dt="boolean">0</a:iscollection>
</a:prop>
</a:propstat>
</a:response></a:multistatus>
In this example, the server responds with a 207 Multi-Status code. WebDAV uses the
207 response for PROPFIND and a few other WebDAV methods that act simulta-
neously on multiple resources and potentially have different responses for each
resource.
A few XML elements in the response need to be defined:
<multistatus>
A container for multiple responses.
<href>
Identifies the resource’s URI.
<status>
Contains the HTTP status code for the particular request.
<propstat>
Groups one <status> element and one <prop> element. The <prop> element
may contain one or more property name/value pairs for the given resource.
In the sample response listed above, the response is for one URI, http://minstar/ch-
publish.fm. The <propstat> element embeds one <status> element and one <prop>
element. For this URI, the server returned a 200 OK response, as defined by the <sta-
tus> element. The <prop> element has several subelements; only some are listed in
the example.
One instant application of PROPFIND is the support for directory listing. Given the
expressability of a PROPFIND request, one single call can retrieve the entire hierar-
chy of the collection with all the properties of individual entities.
The PROPPATCH Method
The PROPPATCH method provides an atomic mechanism to set or remove multiple
properties on a given resource. The atomicity will guarantee that either all of the
requests are successful or none of them made it.
The base XML element for the PROPPATCH method is <propertyupdate>. It acts as
a container for all the properties that need updating. The XML elements <set> and
<remove> are used to specify the operation:
<set>
Specifies the property values to be set. The <set> contains one or more <prop>
subelements, which in turn contains the name/value pairs of the properties to be
set for the resource. If the property already exists, the value is replaced.
WebDAV and Collaborative Authoring |439
<remove>
Specifies the properties that are to be removed. Unlike with <set>, only the
names of the properties are listed in the <prop> container.
This trivial example sets and removes the “owner” property:
<d:propertyupdate xmlns:d="DAV:" xmlns:o="http://name-space/scheme/">
<d:set>
<d:prop>
<o:owner>Author A</o:owner>
</d:prop>
</d:set>
<d:remove>
<d:prop>
<o:owner/>
</d:prop>
</d:remove>
</d:propertyupdate>
The response to PROPPATCH requests is very similar to that for PROPFIND
requests. For more information, refer to RFC 2518.
Table 19-2 summarizes the status codes for the PROPFIND and PROPPATCH
methods.
Collections and Namespace Management
A collection refers to a logical or physical grouping of resources in a predefined hier-
achy. A classic example of a collection is a directory. Like directories in a filesystem,
Table 19-2. Status codes for PROPFIND and PROPPATCH methods
Status code Defined by Methods Effect
200 OK HTTP PROPFIND,
PROPPATCH
Command success.
207 Multi-Status WEBDAV PROPFIND,
PROPPATCH
When acting on one or more resources (or a collection), the status
for each object is encapsulated into one 207 response. This is a
typical success response.
401 Unauthorized HTTP PROPATCH Requires authorization to complete the property modification
operation.
403 Forbidden HTTP PROPFIND,
PROPPATCH
For PROPFIND, the client is not allowed to access the property. For
PROPPATCH, the client may not change the property.
404 Not Found HTTP PROPFIND No such property.
409 Conflict HTTP PROPPATCH Conflict of update semantics—for example, trying to update a
read-only property.
423 Locked WebDAV PROPPATCH Destination resource is locked and there is no lock token or the
lock token does not match.
507 Insufficient Storage WebDAV PROPPATCH Not enough space for registering the modified property.
440 |Chapter 19: Publishing Systems
collections act as containers of other resources, including other collections (equiva-
lent to directories on the filesystem).
WebDAV uses the XML namespace mechanism. Unlike traditional namespaces,
XML namespace partitions allow for precise structural control while preventing any
namespace collisions.
WebDAV provides five methods for manipulating the namespace: DELETE,
MKCOL, COPY, MOVE, and PROPFIND. PROPFIND was discussed previously in
this chapter, but let’s talk about the other methods.
The MKCOL Method
The MKCOL method allows clients to create a collection at the indicated URL on
the server. At first sight, it may seem rather redundant to define an entire new
method just for creating a collection. Overlaying on top of a PUT or POST method
seems like a perfect alternative. The designers of the WebDAV protocol did consider
these alternatives and still chose to define a new method. Some of the reasons behind
that decision are:
• To have a PUT or a POST create a collection, the client needs to send some extra
“semantic glue” along with the request. While this certainly is feasible, defining
an ad hoc protocol may become tedious and error-prone.
• Most of the access-control mechanisms are based on the type of methods—only
a few are allowed to create and delete resources in the repository. If we overload
other methods, these access-control mechanisms will not work.
For example, a request might be:
MKCOL /publishing HTTP/1.1
Host: minstar
Content-Length: 0
Connection: Keep-Alive
And the response might be:
HTTP/1.1 201 Created
Server: Microsoft-IIS/5.0
Date: Fri, 10 May 2002 23:20:36 GMT
Location: http://minstar/publishing/
Content-Length: 0
Let us examine a few pathological cases:
• Suppose the collection already exists. If a MKCOL /colA request is made and
colA already exists (i.e., namespace conflict), the request will fail with a 405
Method Not Allowed status code.
• If there are no write permissions, the MKCOL request will fail with a 403 For-
bidden status code.
WebDAV and Collaborative Authoring |441
• If a request such as MKCOL /colA/colB is made and colA does not exist, the
request will fail with a 409 Conflict status code.
Once the file or collection is created, we can delete it with the DELETE method.
The DELETE Method
We already saw the DELETE method in Chapter 3. WebDAV extends the semantics
to cover collections.
If we need to delete a directory, the Depth header is needed. If the Depth header is
not specified, the DELETE method assumes the Depth header to be set to infinity—
that is, all the files in the directory and any subdirectories thereof are deleted. The
response also has a Content-Location header identifying the collection that just got
deleted. The request might read:
DELETE /publishing HTTP/1.0
Host: minstar
And the response might read:
HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Tue, 14 May 2002 16:41:44 GMT
Content-Location: http://minstar/publishing/
Content-Type: text/xml
Content-Length: 0
When removing collections, there always is a chance that a file in the collection is
locked by someone else and can’t be deleted. In such a case, the collection itself can’t
be deleted, and the server replies with a 207 Multi-Status status code. The request
might read:
DELETE /publishing HTTP/1.0
Host: minstar
And the response might read:
HTTP/1.1 207 Multi-Status
Server: Microsoft-IIS/5.0
Content-Location: http://minstar/publishing/
..............
<?xml version="1.0"?>
<a:multistatus xmlns:a="DAV:">
<a:response>
<a:href>http://minstar/index3/ch-publish.fm</a:href>
<a:status> HTTP/1.1 423 Locked </a:status>
</a:response>
</a:multistatus>
In this transaction, the <status> XML element contains the status code 403 Locked,
indicating that the resource ch-publish.fm is locked by another user.
442 |Chapter 19: Publishing Systems
The COPY and MOVE Methods
As with MKCOL, there are alternatives to defining new methods for COPY and
MOVE operations. One such alternative for the COPY method is to do a GET
request on the source, thus downloading the resource, and then to upload it back to
the server with a PUT request. A similar scenario could be envisioned for MOVE
(with the additional DELETE operation). However, this process does not scale
well—consider all the issues involved in managing a COPY or MOVE operation on a
multilevel collection.
Both the COPY and MOVE methods use the request URL as the source and the con-
tents of the Destination HTTP header as the target. The MOVE method performs
some additional work beyond that of the COPY method: it copies the source URL to
the destination, checks the integrity of the newly created URI, and then deletes the
source. The request might read:
{COPY,MOVE} /publishing HTTP/1.1
Destination: http://minstar/pub-new
Depth: infinity
Overwrite: T
Host: minstar
And the response might read:
HTTP/1.1 201 Created
Server: Microsoft-IIS/5.0
Date: Wed, 15 May 2002 18:29:53 GMT
Location: http://minstar.inktomi.com/pub-new/
Content-Type: text/xml
Content-Length: 0
When acting on a collection, the behavior of COPY or MOVE is affected by the
Depth header. In the absence of the Depth header, infinity is assumed (i.e., by
default, the entire structure of the source directory will be copied or moved). If the
Depth is set to zero, the method is applied just to the resource. If we are doing a copy
or a move of a collection, only a collection with properties identical to those of the
source is created at the destination—no internal members of the collection are cop-
ied or moved.
For obvious reasons, only a Depth value of infinity is allowed with the MOVE
method.
Overwrite header effect
The COPY and MOVE methods also may use the Overwrite header. The Overwrite
header can be set to either Tor F. If it’s set to Tand the destination exists, a DELETE
with a Depth value of infinity is performed on the destination resource before a
COPY or MOVE operation. If the Overwrite flag is set to Fand the destination
resource exists, the operation will fail.
WebDAV and Collaborative Authoring |443
COPY/MOVE of properties
When a collection or an element is copied, all of its properties are copied by default.
However, a request may contain an optional XML body that supplies additional
information for the operation. You can specify that all properties must be copied suc-
cessfully for the operation to succeed, or define which properties must be copied for
the operation to succeed.
A couple of pathological cases to consider are:
• Suppose COPY or MOVE is applied to the output of a CGI program or other
script that generates content. To preserve the semantics, if a file generated by a
CGI script is to be copied or moved, WebDAV provides “src” and “link” XML
elements that point to the location of the program that generated the page.
• The COPY and MOVE methods may not be able to completely duplicate all of
the live properties. For example, consider a CGI program. If it is copied away
from the cgi-bin directory, it may no longer be executed. The current specifica-
tion of WebDAV makes COPY and MOVE a “best effort” solution, copying all
the static properties and the appropriate live properties.
Locked resources and COPY/MOVE
If a resource currently is locked, both COPY and MOVE are prohibited from mov-
ing or duplicating the lock at the destination. In both cases, if the destination is to be
created under an existing collection with its own lock, the duplicated or moved
resource is added to the lock. Consider the following example:
COPY /publishing HTTP/1.1
Destination: http://minstar/archived/publishing-old
Let’s assume that /publishing and /archived already are under two different locks,
lock1 and lock2. When the COPY operation completes, /publishing continues to be
under the scope of lock1, while, by virtue of moving into a collection that’s already
locked by lock2, publishing-old gets added to lock2. If the operation was a MOVE,
just publishing-old gets added to lock2.
Table 19-3 lists most of the possible status codes for the MKCOL, DELETE, COPY,
and MOVE methods.
Table 19-3. Status codes for the MKCOL, DELETE, COPY, and MOVE methods
Status code Defined by Methods Effect
102 Processing WebDAV MOVE,
COPY
If the request takes longer than 20 seconds, the server sends
this status code to keep clients from timing out. This usually is
seen with a COPY or MOVE of a large collection.
201 Created HTTP MKCOL,
COPY,
MOVE
For MKCOL, a collection has been created. For COPY and MOVE,
a resource/collection was copied or moved successfully.
444 |Chapter 19: Publishing Systems
Enhanced HTTP/1.1 Methods
WebDAV modifies the semantics of the HTTP methods DELETE, PUT, and
OPTIONS. Semantics for the GET and HEAD methods remain unchanged. Opera-
tions performed by POST always are defined by the specific server implementation,
and WebDAV does not modify any of the POST semantics. We already covered the
DELETE method, in “Collections and Namespace Management.” We’ll discuss the
PUT and OPTIONS methods here.
The PUT method
Though PUT is not defined by WebDAV, it is the only way for an author to trans-
port the content to a shared site. We discussed the general functionality of PUT in
Chapter 3. WebDAV modifies its behavior to support locking.
204 No Content HTTP DELETE,
COPY,
MOVE
For DELETE, a standard success response. For COPY and MOVE,
the resource was copied over successfully or moved to replace
an existing entity.
207 Multi-Status WebDAV MKCOL,
COPY,
MOVE
For MKCOL, a typical success response. For COPY and MOVE, if
an error is associated with a resource other than the request
URI, the server returns a 207 response with the XML body
detailing the error.
403 Forbidden HTTP MKCOL,
COPY,
MOVE
For MKCOL, the server does not allow creation of a collection at
the specified location. For COPY and MOVE, the source and
destination are the same.
409 Conflict HTTP MKCOL,
COPY,
MOVE
In all cases, the methods are trying to create a collection or a
resource when an intermediate collection does not exist—for
example, trying to create colA/colB when colA does not exist.
412 Precondition Failed HTTP COPY,
MOVE
Either the Overwrite header is set to F and the destination
exists, or the XML body specifies a certain requirement (such
as keeping the “liveness” property) and the COPY or MOVE
methods are not able to retain the property.
415 Unsupported Media Type HTTP MKCOL The server does not support or understand the creation of the
request entity type.
422 Unprocessable Entity WebDAV MKCOL The server does not understand the XML body sent with the
request.
423 Locked WebDAV DELETE,
COPY,
MOVE
The source or the destination resource is locked, or the lock
token supplied with the method does not match.
502 Bad Gateway HTTP COPY,
MOVE
The destination is on a different server and permissions are
missing.
507 Insufficient Storage WebDAV MKCOL
COPY
There is not enough free space to create the resource.
Table 19-3. Status codes for the MKCOL, DELETE, COPY, and MOVE methods (continued)
Status code Defined by Methods Effect
WebDAV and Collaborative Authoring |445
Consider the following example:
PUT /ch-publish.fm HTTP/1.1
Accept: */*
If:<http://minstar/index.htm>(<opaquelocktoken:********>)
User-Agent: DAV Client (C)
Host: minstar.inktomi.com
Connection: Keep-Alive
Cache-Control: no-cache
Content-Length: 1155
To support locking, WebDAV adds an If header to the PUT request. In the above
transaction, the semantics of the If header state that if the lock token specified with
the If header matches the lock on the resource (in this case, ch-publish.fm), the PUT
operation should be performed. The If header also is used with a few other methods,
such as PROPPATCH, DELETE, MOVE, LOCK, UNLOCK, etc.
The OPTIONS method
We discussed OPTIONS in Chapter 3. This usually is the first request a WebDAV-
enabled client makes. Using the OPTIONS method, the client tries to establish the
capability of the WebDAV server. Consider a transaction in which the request reads:
OPTIONS /ch-publish.fm HTTP/1.1
Accept: */*
Host: minstar.inktomi.com
And the response reads:
HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
MS-Author-Via: DAV
DASL: <DAV:sql>
DAV: 1, 2
Public: OPTIONS, TRACE, GET, HEAD, DELETE, PUT, POST, COPY, MOVE, MKCOL,PROPFIND,
PROPPATCH, LOCK, UNLOCK, SEARCH
Allow: OPTIONS, TRACE, GET, HEAD, DELETE, PUT, COPY, MOVE, PROPFIND,PROPPATCH,
SEARCH, LOCK, UNLOCK
There are several interesting headers in the response to the OPTIONS method. A
slightly out-of-order examination follows:
• The DAV header carries the information about DAV compliance classes. There
are two classes of compliance:
Class 1 compliance
Requires the server to comply with all MUST requirements in all sections of
RFC 2518. If the resource complies only at the Class 1 level, it will send 1
with the DAV header.
Class 2 compliance
Meets all the Class 1 requirements and adds support for the LOCK method.
Along with LOCK, Class 2 compliance requires support for the Timeout and
446 |Chapter 19: Publishing Systems
Lock-Token headers and the <supportedlock> and <lockdiscovery> XML
elements. A value of 2 in the DAV header indicates Class 2 compliance.
In the above example, the DAV header indicates both Class 1 and Class 2
compliance.
• The Public header lists all methods supported by this particular server.
• The Allow header usually contains a subset of the Public header methods. It lists
only those methods that are allowed on this particular resource (ch-publish.fm).
• The DASL header provides the type of query grammar used in the SEARCH
method. In this case, it is sql. More details about the DASL header are provided
at http://www.webdav.org.
Version Management in WebDAV
It may be ironic, given the “V” in “DAV,” but versioning is a feature that did not
make the first cut. In a multi-author, collaborative environment, version manage-
ment is critical. In fact, to completely fix the lost update problem (illustrated in
Figure 19-3), locking and versioning are essential. Some of the common features
associated with versioning are the ability to store and access previous document ver-
sions and the ability to manage the change history and any associated annotations
detailing the changes.
Versioning was added to WebDAV in RFC 3253.
Future of WebDAV
WebDAV is well supported today. Working implementations of clients include IE 5.
x and above, Windows Explorer, and Microsoft Office. On the server side, imple-
mentations include IIS5.x and above, Apache with mod_dav, and many others. Both
Windows XP and Mac OS 10.x provide support for WebDAV out of the box; thus,
any applications written to run on these operating systems are WebDAV-enabled
natively.
For More Information
For more information, refer to:
http://officeupdate.microsoft.com/frontpage/wpp/serk/
Microsoft FrontPage 2000 Server Extensions Resource Kit.
http://www.ietf.org/rfc/rfc2518.txt?number=2518
“HTTP Extensions for Distributed Authoring—WEBDAV,” by Y. Goland, J.
Whitehead, A. Faizi, S. Carter, and D. Jensen.
For More Information |447
http://www.ietf.org/rfc/rfc3253.txt?number=3253
“Versioning Extensions to WebDAV,” by G. Clemm, J. Amsden, T. Ellison, C.
Kaler, and J. Whitehead.
http://www.ics.uci.edu/pub/ietf/webdav/intro/webdav_intro.pdf
“WEBDAV: IETF Standard for Collaborative Authoring on the Web,” by J.
Whitehead and M. Wiggins.
http://www.ics.uci.edu/~ejw/http-future/whitehead/http_pos_paper.html
“Lessons from WebDAV for the Next Generation Web Infrastructure,” by J.
Whitehead.
http://www.microsoft.com/msj/0699/dav/davtop.htm
“Distributed Authoring and Versioning Extensions for HTTP Enable Team
Authoring,” by L. Braginski and M. Powell.
http://www.webdav.org/dasl/protocol/draft-dasl-protocol-00.html
“DAV Searching & Locating,” by S. Reddy, D. Lowry, S. Reddy, R. Henderson,
J. Davis, and A. Babich.
448
CHAPTER 20
Redirection and Load Balancing
HTTP does not walk the Web alone. The data in an HTTP message is governed by
many protocols on its journey. HTTP cares only about the endpoints of the journey—
the sender and the receiver—but in a world with mirrored servers, web proxies, and
caches, the destination of an HTTP message is not necessarily straightforward.
This chapter is about redirection technologies—network tools, techniques, and pro-
tocols that determine the final destination of an HTTP message. Redirection technol-
ogies usually determine whether the message ends up at a proxy, a cache, or a
particular web server in a server farm. Redirection technologies may send your mes-
sages to places a client didn’t explicitly request.
In this chapter, we’ll take a look at the following redirection techniques, how they
work, and what their load-balancing capabilities are (if any):
• HTTP redirection
• DNS redirection
• Anycast routing
• Policy routing
• IP MAC forwarding
• IP address forwarding
• The Web Cache Coordination Protocol (WCCP)
• The Intercache Communication Protocol (ICP)
• The Hyper Text Caching Protocol (HTCP)
• The Network Element Control Protocol (NECP)
• The Cache Array Routing Protocol (CARP)
• The Web Proxy Autodiscovery Protocol (WPAD)
Where to Redirect |449
Why Redirect?
Redirection is a fact of life in the modern Web because HTTP applications always
want to do three things:
• Perform HTTP transactions reliably
• Minimize delay
• Conserve network bandwidth
For these reasons, web content often is distributed in multiple locations. This is done
for reliability, so that if one location fails, another is available; it is done to lower
response times, because if clients can access a nearer resource, they receive their
requested content faster; and it’s done to lower network congestion, by spreading
out target servers. You can think of redirection as a set of techniques that help to find
the “best” distributed content.
The subject of load balancing is included because redirection and load balancing
coexist. Most redirection deployments include some form of load balancing; that is,
they are capable of spreading incoming message load among a set of servers. Con-
versely, any form of load balancing involves redirection, because incoming messages
must somehow be somehow among the servers sharing the load.
Where to Redirect
Servers, proxies, caches, and gateways all appear to clients as servers, in the sense
that a client sends them an HTTP request, and they process it. Many redirection
techniques work for servers, proxies, caches, and gateways because of their com-
mon, server-like traits. Other redirection techniques are specially designed for a par-
ticular class of endpoint and are not generally applicable. We’ll see general
techniques and specialized techniques in later sections of this chapter.
Web servers handle requests on a per-IP basis. Distributing requests to duplicate
servers means that each request for a specific URL should be sent to an optimal web
server (the one nearest to the client, or the least-loaded one, or some other optimiza-
tion). Redirecting to a server is like sending all drivers in search of gasoline to the
nearest gas station.
Proxies tend to handle requests on a per-protocol basis. Ideally, all HTTP traffic in the
neighborhood of a proxy should go through the proxy. For instance, if a proxy cache
is near various clients, all requests ideally will flow through the proxy cache, because
the cache will store popular documents and serve them directly, avoiding longer and
more expensive trips to the origin servers. Redirecting to a proxy is like siphoning off
traffic on a main access road (no matter where it is headed) to a local shortcut.
450 |Chapter 20: Redirection and Load Balancing
Overview of Redirection Protocols
The goal of redirection is to send HTTP messages to available web servers as quickly
as possible. The direction that an HTTP message takes on its way through the Inter-
net is affected by the HTTP applications and routing devices it passes from, through,
and toward. For example:
• The browser application that creates the client’s message could be configured to
send it to a proxy server.
• DNS resolvers choose the IP address that is used for addressing the message.
This IP address can be different for different clients in different geographical
locations.
• As the message passes through networks, it is divided into addressed packets;
switches and routers examine the TCP/IP addressing on the packets and make
decisions about routing the packets on that basis.
• Web servers can bounce requests back to different web servers with HTTP
redirects.
Browser configuration, DNS, TCP/IP routing, and HTTP all provide mechanisms for
redirecting messages. Notice that some methods, such as browser configuration,
make sense only for redirecting traffic to proxies, while others, such as DNS redirec-
tion, can be used to send traffic to any server.
Table 20-1 summarizes the redirection methods used to redirect messages to servers,
each of which is discussed later in this chapter.
Table 20-1. General redirection methods
Mechanism How it works Basis for rerouting Limitations
HTTP redirection Initial HTTP request goes to a first web
server that chooses a “best” web server
to serve the content. The first web
server sends the client an HTTP redirect
to the chosen server. The client resends
the request to the chosen server.
Many options, from
round-robin load
balancing,to minimizing
latency, to choosing the
shortest path.
Can be slow—every trans-
action involves the extra
redirect step. Also, the first
server must be able to han-
dle the request load.
DNS redirection DNS server decides which IP address,
among several, to return for the host-
name in the URL.
Many options, from
round-robin load
balancing,to minimizing
latency, to choosing the
shortest path.
Need to configure DNS
server.
Anycast addressing Several servers use the same IP address.
Each server masquerades as a backbone
router. The other routers send packets
addressed to the shared IP to the near-
est server (believing they are sending
packets to the nearest router).
Routers use built-in
shortest-path routing
capabilities.
Need to own/configure
routers. Risks address con-
flicts. Established TCP con-
nectionscan break if routing
changes and packets associ-
ated with a connection get
sent to different servers.
Overview of Redirection Protocols |451
Table 20-2 summarizes the redirection methods used to redirect messages to proxy
servers.
IP MAC forwarding A network element such as a switch or
router reads a packet’s destination
address; if the packet should be redi-
rected, the switch gives the packet the
destination MAC address of a server or
proxy.
Save bandwidth and
improve QOS. Load
balance.
Server or proxy must be one
hop away.
IP address
forwarding
Layer-4 switch evaluates a packet’s des-
tination port and changes the IP address
of a redirect packet to that of a proxy or
mirrored server.
Save bandwidth and
improve QOS. Load
balance.
IP address of the client can
be lost to the server/proxy.
Table 20-2. Proxy and cache redirection techniques
Mechanism How it works Basis for rerouting Limitations
Explicit browser
configuration
Web browser is configured to send HTTP
messages to a nearby proxy, usually a
cache. The configuration can be done by
the end user or by a service that man-
ages the browser.
Save bandwidth and
improve QOS. Load
balance.
Depends on ability to con-
figure the browser.
Proxy auto-
configuration (PAC)
Web browser retrieves a PAC file from a
configurationserver. The PACfiletells the
browser what proxy to use for each URL.
Save bandwidth and
improve QOS. Load
balance.
Browser must be config-
ured to query the configura-
tion server.
Web Proxy
Autodiscovery
Protocol (WPAD)
Web browser asks a configuration server
for the URL of a PAC file. Unlike PAC
alone, the browser does not have to be
configured with a specific configuration
server.
The configuration server
bases the URL on infor-
mation in client HTTP
request headers. Load
balance.
Onlya few browsers support
WPAD.
Web Cache
Coordination
Protocol (WCCP)
Router evaluates a packet’s destination
address and encapsulates redirect pack-
ets with the IP address of a proxy or mir-
rored server. Works with many existing
routers. Packet can be encapsulated, so
the client’s IP address is not lost.
Save bandwidth and
improve QOS. Load
balance.
Must use routers that sup-
port WCCP. Some topologi-
cal limitations.
Internet Cache
Protocol (ICP)
A proxy cache can query a group of sib-
ling caches for requested content. Also
supports cache hierarchies.
Obtaining content from
a sibling or parent cache
is faster than applying to
the origin server.
False cache hits can arise
becauseonly the URL is used
to request content.
Cache Array Rout-
ing Protocol (CARP)
Aproxycachehashingprotocol. Allows a
cache to forward a request to a parent
cache. Unlike with ICP, the content on
the caches is disjoint, and the group of
caches acts as a single large cache.
Obtaining content from
a nearby peer cache is
faster than applying to
the origin server.
CARP cannot support sib-
ling relationships. All CARP
clients must agree on the
configuration; otherwise,
different clients will send
the same URI to different
parents, reducing hit ratios.
Table 20-1. General redirection methods (continued)
Mechanism How it works Basis for rerouting Limitations
452 |Chapter 20: Redirection and Load Balancing
General Redirection Methods
In this section, we will delve deeper into the various redirection methods that are
commonly used for both servers and proxies. These techniques can be used to redi-
rect traffic to a different (presumably more optimal) server or to vector traffic
through a proxy. Specifically, we’ll cover HTTP redirection, DNS redirection, any-
cast addressing, IP MAC forwarding, and IP address forwarding.
HTTP Redirection
Web servers can send short redirect messages back to clients, telling them to try
someplace else. Some web sites use HTTP redirection as a simple form of load bal-
ancing; the server that handles the redirect (the redirecting server) finds the least-
loaded content server available and redirects the browser to that server. For widely
distributed web sites, determining the “best” available server gets more complicated,
taking into account not only the servers’ load but the Internet distance between the
browser and the server. One advantage of HTTP redirection over some other forms
of redirection is that the redirecting server knows the client’s IP address; in theory, it
may be able to make a more informed choice.
Here’s how HTTP redirection works. In Figure 20-1a, Alice sends a request to
www.joes-hardware.com:
GET /hammers.html HTTP/1.0
Host: www.joes-hardware.com
User-Agent: Mozilla/4.51 [en] (X11; U; IRIX 6.2 IP22)
In Figure 20-1b, instead of sending back a web page body with HTTP status code
200, the server sends back a redirect message with status code 302:
HTTP/1.0 302 Redirect
Server: Stronghold/2.4.2 Apache/1.3.6
Location: http://161.58.228.45/hammers.html
Now, in Figure 20-1c, the browser resends the request using the redirected URL, this
time to host 161.58.228.45:
GET /hammers.html HTTP/1.0
Host: 161.58.228.45
User-Agent: Mozilla/4.51 [en] (X11; U; IRIX 6.2 IP22)
Another client could get redirected to a different server. In Figure 20-1d–f, Bob’s
request gets redirected to 161.58.228.46.
Hyper Text Caching
Protocol (HTCP)
Participating proxy caches can query a
group of sibling caches for requested
content. Supports HTTP 1.0 and 1.1
headers to fine-tune cache queries.
Obtaining content from
a sibling or parent cache
is faster than applying to
the origin server.
Table 20-2. Proxy and cache redirection techniques (continued)
Mechanism How it works Basis for rerouting Limitations
General Redirection Methods |453
HTTP redirection can vector requests across servers, but it has several disadvantages:
• A significant amount of processing power is required from the original server to
determine which server to redirect to. Sometimes almost as much server horse-
power is required to issue the redirect as would be to serve up the page itself.
• User delays are increased, because two round trips are required to access pages.
• If the redirecting server is broken, the site will be broken.
Because of these weaknesses, HTTP redirection usually is used in combination with
some of the other redirection technique.
DNS Redirection
Every time a client tries to access Joe’s Hardware’s web site, the domain name
www.joes-hardware.com must be resolved to an IP address. The DNS resolver may
be the client’s own operating system, a DNS server in the client’s network, or a
more remote DNS server. DNS allows several IP addresses to be associated to a sin-
gle domain, and DNS resolvers can be configured or programmed to return varying
IP addresses. The basis on which the resolver returns the IP address can run from
the simple (round robin) to the complex (such as checking the load on several serv-
ers and returning the IP address of the least-loaded server).
Figure 20-1. HTTP redirection
(c
)
B
ro
w
s
e
rr
ese
n
dsH
TT
Preques
t,thistime
to161
.5
8.
228
.4
5
(f
)
B
row
s
err
ese
nds HTT
Prequest,this timeto
161.5
8.228
.4
6
Internet
Bob
Alice www.joes-hardware.com
161.58.228.45
(a) Alice sends HTTP request to www.joes.hardware.com
(b) Server returns 302 redirect to 161.58.228.45
161.58.228.46 161.58.228.47
Internet
Bob
Alice
www.joes-hardware.com
161.58.228.45 161.58.228.46 161.58.228.47
(d) Bob sends HTTP request to www.joes-hardware.com
(e) Server returns 302 redirect to 161.58.228.46
454 |Chapter 20: Redirection and Load Balancing
In Figure 20-2, Joe runs four servers for www.joes-hardware.com. The DNS server
has to decide which of four IP addresses to return for www.joes-hardware.com. The
easiest DNS decision algorithm is a simple round robin.
For a run-through of the DNS resolution process, see the DNS reference listed at the
end of this chapter.
DNS round robin
One of the most common redirection techniques also is one of the simplest. DNS
round robin uses a feature of DNS hostname resolution to balance load across a farm
of web servers. It is a pure load-balancing strategy, and it does not take into account
any factors about the location of the client relative to the server or the current stress
on the server.
Let’s look at what CNN.com really does. In early May of 2000, we used the nslookup
Unix tool to find the IP addresses associated with CNN.com. Example 20-1 shows
the results.*
Figure 20-2. DNS-based redirection
* DNS results as of May 7, 2000 and resolved from Northern California. The particular values likely will
change over time, and some DNS systems return different values based on client location.
Example 20-1. IP addresses for www.cnn.com
%nslookup www.cnn.com
Name: cnn.com
10.10.10.4
Server 4
www.joes-hardware.com
10.10.10.1
Server 1
www.joes-hardware.com
10.10.10.2
Server 2
www.joes-hardware.com
10.10.10.3
Server 3
www.joes-hardware.com
Decides whether
to resolve to
10.10.10.1,
10.10.10.2,
10.10.10.3,
10.10.10.4
DNS server
Edge network
Client
Client
Switch Router
Backbone network
General Redirection Methods |455
The web site www.cnn.com actually is a farm of 20 distinct IP addresses! Each IP
address might typically translate to a different physical server.
Multiple addresses and round-robin address rotation
Most DNS clients just use the first address of the multi-address set. To balance load,
most DNS servers rotate the addresses each time a lookup is done. This address rota-
tion often is called DNS round robin.
For example, three consecutive DNS lookups of www.cnn.com might return rotated
lists of IP addresses like those shown in Example 20-2.
In Example 20-2:
• The first address of the first DNS lookup is 207.25.71.5.
• The first address of the second DNS lookup is 207.25.71.6.
• The first address of the third DNS lookup is 207.25.71.7.
Addresses: 207.25.71.5, 207.25.71.6, 207.25.71.7, 207.25.71.8
207.25.71.9, 207.25.71.12, 207.25.71.20, 207.25.71.22, 207.25.71.23
207.25.71.24, 207.25.71.25, 207.25.71.26, 207.25.71.27, 207.25.71.28
207.25.71.29, 207.25.71.30, 207.25.71.82, 207.25.71.199, 207.25.71.245
207.25.71.246
Aliases: www.cnn.com
Example 20-2. Rotating DNS address lists
%nslookup www.cnn.com
Name: cnn.com
Addresses: 207.25.71.5, 207.25.71.6, 207.25.71.7, 207.25.71.8
207.25.71.9, 207.25.71.12, 207.25.71.20, 207.25.71.22, 207.25.71.23
207.25.71.24, 207.25.71.25, 207.25.71.26, 207.25.71.27, 207.25.71.28
207.25.71.29, 207.25.71.30, 207.25.71.82, 207.25.71.199, 207.25.71.245
207.25.71.246
%nslookup www.cnn.com
Name: cnn.com
Addresses: 207.25.71.6, 207.25.71.7, 207.25.71.8, 207.25.71.9
207.25.71.12, 207.25.71.20, 207.25.71.22, 207.25.71.23, 207.25.71.24
207.25.71.25, 207.25.71.26, 207.25.71.27, 207.25.71.28, 207.25.71.29
207.25.71.30, 207.25.71.82, 207.25.71.199, 207.25.71.245, 207.25.71.246
207.25.71.5
%nslookup www.cnn.com
Name: cnn.com
Addresses: 207.25.71.7, 207.25.71.8, 207.25.71.9, 207.25.71.12
207.25.71.20, 207.25.71.22, 207.25.71.23, 207.25.71.24, 207.25.71.25
207.25.71.26, 207.25.71.27, 207.25.71.28, 207.25.71.29, 207.25.71.30
207.25.71.82, 207.25.71.199, 207.25.71.245, 207.25.71.246, 207.25.71.5
207.25.71.6
Example 20-1. IP addresses for www.cnn.com (continued)
456 |Chapter 20: Redirection and Load Balancing
DNS round robin for load balancing
Because most DNS clients just use the first address, the DNS rotation serves to bal-
ance load among servers. If DNS did not rotate the addresses, most clients would
always send load to the first client.
Figure 20-3 shows how DNS round-robin rotation acts to balance load:
• When Alice tries to connect to www.cnn.com, she looks up the IP address using
DNS and gets back 207.25.71.5 as the first IP address. Alice connects to the web
server 207.25.71.5 in Figure 20-3c.
• When Bob subsequently tries to connect to www.cnn.com, he also looks up the
IP address using DNS, but he gets back a different result because the address list
has been rotated one position, based on Alice’s previous request. Bob gets back
207.25.71.6 as the first IP address, and he connects to this server in Figure 20-3f.
The impact of DNS caching
DNS address rotation spreads the load around, because each DNS lookup to a server
gets a different ordering of server addresses. However, this load balancing isn’t per-
fect, because the results of the DNS lookup may be memorized and reused by applica-
tions, operating systems, and some primitive child DNS servers. Many web browsers
Figure 20-3. DNS round robin load balances across servers in a server farm
(c
)
A
li
ce
s
e
n
dsH
T
TPr
equ
estto20
7.25.71.5
(f
)
B
ob
se
nds
HT
TPrequ
estto 207.25.71.6
Internet
Bob
Alice DNS server
207.25.71.5
(a) Alice asks DNS for IP address of www.cnn.com
(b) DNS replies with 207.25.71.5
207.25.71.6 207.25.71.7
Internet
Bob
Alice
DNS server
207.25.71.5 207.25.71.6 207.25.71.7
(d) Bob asks DNS for IP address of www.cnn.com
(e) DNS replies with 207.25.71.6
General Redirection Methods |457
perform a DNS lookup for a host but then use the same address over and over again,
to eliminate the cost of DNS lookups and because some servers prefer to keep talking
to the same client. Furthermore, many operating systems perform the DNS lookup
automatically, and cache the result, but don’t rotate the addresses. Consequently,
DNS round robin generally doesn’t balance the load of a single client—one client typ-
ically will be stuck to one server for a long period of time.
But, even though DNS doesn’t deal out the transactions of a single client across
server replicas, it does a decent job of spreading the aggregate load of multiple cli-
ents. As long as there is a modestly large number of clients with similar demand, the
load will be relatively well distributed across servers.
Other DNS-based redirection algorithms
We’ve already discussed how DNS rotates address lists with each request. However,
some enhanced DNS servers use other techniques for choosing the order of the
addresses:
Load-balancing algorithms
Some DNS servers keep track of the load on the web servers and place the least-
loaded web servers at the front of the list.
Proximity-routing algorithms
DNS servers can attempt to direct users to nearby web servers, when the farm of
web servers is geographically dispersed.
Fault-masking algorithms
DNS servers can monitor the health of the network and route requests away
from service interruptions or other faults.
Typically, the DNS server that runs sophisticated server-tracking algorithms is an
authoritative server that is under the control of the content provider (see Figure 20-4).
Several distributed hosting services use this DNS redirection model. One drawback
of the model for services that look for nearby servers is that the only information that
the authoritative DNS server uses to make its decision is the IP address of the local
DNS server, not the IP address of the client.
Anycast Addressing
In anycast addressing, several geographically dispersed web servers have the exact
same IP address and rely on the “shortest-path” routing capabilities of backbone
routers to send client requests to the server nearest to the client. One way this
method can work is for each web server to advertise itself as a router to a neighbor-
ing backbone router. The web server talks to its neighboring backbone router using a
router communication protocol. When the backbone router receives packets aimed
at the anycast address, it looks (as it usually would) for the nearest “router” that
458 |Chapter 20: Redirection and Load Balancing
accepts that IP address. Because the server will have advertised itself as a router for
that address, the backbone router will send the server the packet.
In Figure 20-5, three servers front the same IP address, 10.10.10.1. The Los Angeles
(LA) server advertises this address to the LA router, the New York (NY) server adver-
tises the same address to the NY router, and so on. The servers communicate with
the routers using a router protocol. The routers automatically route client requests
aimed at 10.10.10.1 to the nearest server that advertises the address. In Figure 20-5,
a request for the IP address 10.10.10.1 will be routed to server 3.
Figure 20-4. DNS request involving authoritative server
Figure 20-5. Distributed anycast addressing
(f)SendHTTPrequesttoserverat207.25.71.5
Alice
Local DNS server
(a) Request IP address for www.cnn.com
Root DNS server
(b) Request IP address from root server
(c) Sends IP address of authoritive server
(d) Request IP address
(e) Returns IP address 207.25.71.5
Authoritative DNS server
monitors cnn servers
www.cnn.com
(207.25.71.5) www.cnn.com
(207.25.71.6) www.cnn.com
(207.25.71.7)
10.10.10.1
Server 1
www.joes-hardware.com
10.10.10.1
Server 2
www.joes-hardware.com
10.10.10.1
Server 3
www.joes-hardware.com
Edge network
Client
Client
Switch Router
Backbone network
Router
Router
General Redirection Methods |459
Anycast addressing is still an experimental technique. For distributed anycast to
work, the servers must “speak router language” and the routers must be able to han-
dle possible address conflicts, because Internet addressing basically assumes one
server for one address. (If done improperly, this can lead to serious problems known
as “route leaks.”) Distributed anycast is an emerging technology and might be a solu-
tion for content providers who control their own backbone networks.
IP MAC Forwarding
In Ethernet networks, HTTP messages are sent in the form of addressed data pack-
ets. Each packet has a layer-4 address, consisting of the source and destination IP
address and TCP port numbers; this is the address to which layer 4–aware devices
pay attention. Each packet also has a layer-2 address, the Media Access Control
(MAC) address, to which layer-2 devices (commonly switches and hubs) pay atten-
tion. The job of layer-2 devices is to receive packets with particular incoming MAC
addresses and forward them to particular outgoing MAC addresses.
In Figure 20-6, for example, the switch is programmed to send all traffic from MAC
address “MAC3” to MAC address “MAC4.”
A layer 4–aware switch is able to examine the layer-4 addressing (IP addresses and
TCP port numbers) and make routing decisions based on this information. For
example, a layer-4 switch could send all port 80–destined web traffic to a proxy. In
Figure 20-7, the switch is programmed to send all port 80 traffic from MAC3 to
MAC6 (a proxy cache). All other MAC3 traffic goes to MAC5.
Typically, if the requested HTTP content is in the cache and is fresh, the proxy cache
serves it; otherwise, the proxy cache sends an HTTP request to the origin server for
the content, on the client’s behalf. The switch sends port 80 requests from the proxy
(MAC6) to the Internet gateway (MAC5).
Layer-4 switches that support MAC forwarding usually can forward requests to sev-
eral proxy caches and balance the load among them. Likewise, HTTP traffic also can
be forwarded to alternate HTTP servers.
Figure 20-6. Layer-2 switch sending client requests to a gateway
Client MAC1
Client MAC2
Switch MAC4Hub MAC3
Gateway MAC5
To Internet
460 |Chapter 20: Redirection and Load Balancing
Because MAC address forwarding is point-to-point only, the server or proxy has to
be located one hop away from the switch.
IP Address Forwarding
In IP address forwarding, a switch or other layer 4–aware device examines TCP/IP
addressing on incoming packets and routes packets accordingly by changing the des-
tination IP address, instead of the destination MAC address. An advantage over
MAC forwarding is that the destination server need not be one hop away; it just
needs to be located upstream from the switch, and the usual layer-3 end-to-end
Internet routing gets the packet to the right place. This type of forwarding also is
called Network Address Translation (NAT).
There is a catch, however: routing symmetry. The switch that accepts the incoming
TCP connection from the client is managing that connection; the switch must send
the response back to the client on that TCP connection. Therefore, any response
from the destination server or proxy must return to the switch (see Figure 20-8).
Figure 20-7. MAC forwarding using a layer-4 switch
Figure 20-8. A switch doing IP forwarding to a caching proxy or mirrored web server
Client MAC1
Client MAC2
Switch MAC4Hub MAC3
Gateway MAC5
To Internet
Caching proxy MAC6
Port 80 traffic
Joe's server
Client edge
network
Client
Client
Switch Router
Backbone network
Destination
proxy
Joe's edge
network
General Redirection Methods |461
Two ways to control the return path of the response are:
• Change the source IP address of the packet to the IP address of the switch. That
way, regardless of the network configuration between the switch and server, the
response packet goes to the switch. This is called full NAT, where the IP for-
warding device translates both destination and source IP addresses. Figure 20-9
shows the effect of full NAT on a TCP/IP datagram. The consequence is that the
client IP address is unknown to the web server, which might want it for authenti-
cation or billing purposes, for example.
• If the source IP address remains the client’s IP address, make sure (from a hard-
ware perspective) that no routes exist directly from server to client (bypassing
the switch). This sometimes is called half NAT. The advantage here is that the
server obtains the client IP address, but the disadvantage is the requirement of
some control of the entire network between client and server.
Network Element Control Protocol
The Network Element Control Protocol (NECP) allows network elements (NEs)—
devices such as routers and switches that forward IP packets—to talk with server ele-
ments (SEs)—devices such as web servers and proxy caches that serve application
layer requests. NECP does not explicitly support load balancing; it only offers a way
for an SE to send an NE load-balancing information so that the NE can load balance
as it sees fit. Like WCCP, NECP offers several ways to forward packets: MAC for-
warding, GRE encapsulation, and NAT.
NECP supports the idea of exceptions. The SE can decide that it cannot service par-
ticular source IP addresses, and send those addresses to the NE. The NE can then
forward requests from those IP addresses to the origin server.
Messages
The NECP messages are described in Table 20-3.
Figure 20-9. Full NAT of a TCP/IP datagram
From:Client:
1.1.1.1
80
Passes through network address
translation (NAT) device
To:Joe’s Server:
2.2.2.2
80
From:NAT device:
3.3.3.3
80 To:Proxy:
4.4.4.4
80
HTTP data
HTTP data
462 |Chapter 20: Redirection and Load Balancing
Proxy Redirection Methods
So far, we have talked about general redirection methods. Content also may need to
be accessed through various proxies (potentially for security reasons), or there might
be a proxy cache in the network that a client should take advantage of (because it
likely will be much faster to retrieve the cached content than it would be to go
directly to the origin server).
But how do clients such as web browsers know to go to a proxy? There are three
ways to determine this: by explicit browser configuration, by dynamic automatic
configuration, and by transparent interception. We will discuss these three tech-
niques in this section.
A proxy can, in turn, redirect client requests to a different proxy. For example, a proxy
cache that does not have the content in its cache may choose to redirect the client to
another cache. As this results in the response coming from a location different from
the one from which the client requested the resource, we also will discuss several pro-
tocols used for peer proxy-cache redirection: the Internet Cache Protocol (ICP), the
Cache Array Routing Protocol (CARP), and the Hyper Text Caching Protocol (HTCP).
Table 20-3. NECP messages
Message Who sends it Meaning
NECP_NOOP No operation—do nothing.
NECP_INIT SE SE initiates communication with NE. SE sends this message to NE after
opening TCP connection with NE. SE must know which NE port to con-
nect to.
NECP_INIT_ACK NE Acknowledges NECP_INIT.
NECP_KEEPALIVE NE or SE Asks if peer is alive.
NECP_KEEPALIVE_ACK NE or SE Answers keep-alive message.
NECP_START SE SE says “I am here and ready to accept network traffic.” Can specify a
port.
NECP_START_ACK NE Acknowledges NECP_START.
NECP_STOP SE SE tells NE “stop sending me traffic.”
NECP_STOP_ACK NE NE acknowledges stop.
NECP_EXCEPTION_ADD SE SE says to add one or more exceptions to NE’s list. Exceptions can be
based on source IP, destination IP, protocol (above IP), or port.
NECP_EXCEPTION_ADD_ACK NE Confirms EXCEPTION_ADD.
NECP_EXCEPTION_DEL SE Asks NE to delete one or more exceptions from its list.
NECP_EXCEPTION_DEL_ACK NE Confirms EXCEPTION_DEL.
NECP_EXCEPTION_RESET SE Asks NE to delete entire exception list.
NECP_EXCEPTION_RESET_ACK NE Confirms EXCEPTION_RESET.
NECP_EXCEPTION_QUERY SE Queries NE’s entire exception list.
NECP_EXCEPTION_RESP NE Responds to exception query.
Proxy Redirection Methods |463
Explicit Browser Configuration
Most browsers can be configured to contact a proxy server for content—there is a
pull-down menu where the user can enter the proxy’s name or IP address and port
number. The browser then contacts the proxy for all requests. Rather than relying on
users to correctly configure their browsers to use proxies, some service providers
require users to download preconfigured browsers. These browsers know the address
of the proxy to contact.
Explicit browser configuration has two main disadvantages:
• Browsers configured to use proxies do not contact the origin server even if the
proxy is not responding. If the proxy is down or if the browser is incorrectly con-
figured, the user experiences connectivity problems.
• It is difficult to make changes in network architecture and propagate those
changes to all end users. If a service provider wants to add more proxies or take
some out of service, browser users have to change their proxy settings.
Proxy Auto-configuration
Explicit configuration of browsers to contact specific proxies can restrict changes in
network architecture, because it depends on users to intervene and reconfigure their
browsers. An automatic configuration methodology that allows browsers to dynami-
cally configure themselves to contact the correct proxy server solves this problem.
Such a methodology exists; it is called the Proxy Auto-configuration (PAC) protocol.
PAC was defined by Netscape and is supported by the Netscape Navigator and
Microsoft Internet Explorer browsers.
The basic idea behind PAC is to have browsers retrieve a special file, called the PAC
file, which specifies the proxy to contact for each URL. The browser must be config-
ured to contact a specific server for the PAC file. The browser then fetches the PAC
file every time it is restarted.
The PAC file is a JavaScript file, which must define the function:
function FindProxyForURL(url, host)
Browsers call this function for every requested URL, as follows:
return_value = FindProxyForURL(url_of_request, host_in_url);
where the return value is a string specifying where the browser should request this
URL. The return value can be a list of the names of proxies to contact (for example,
“PROXY proxy1.domain.com; PROXY proxy2.domain.com”) or the string
“DIRECT”, which means that the browser should go directly to the origin server,
bypassing any proxies.
The sequence of operations that illustrate the request for and response to a browser’s
request for the PAC file are illustrated in Figure 20-10. In this example, the server
464 |Chapter 20: Redirection and Load Balancing
sends back a PAC file with a JavaScript program. The JavaScript program has a func-
tion called “FindProxyForURL” that tells the browser to contact the origin server
directly if the host in the requested URL is in the “netscape.com” domain, and to go
to “proxy1.joes-cache.com” for all other requests. The browser calls this function for
each URL it requests and connects according to the results returned by the function.
The PAC protocol is quite powerful: the JavaScript program can ask the browser to
choose a proxy based on any of a number of parameters related to the hostname, such
as the DNS address and subnet, and even the day of week or time of day. PAC allows
browsers automatically to contact the right proxy with changes in network architec-
ture, as long as the PAC file is updated at the server to reflect changes to the proxy
locations. The main drawback with PAC is that the browser must be configured to
know which server to fetch the PAC file from, so it is not a completely automatic con-
figuration system. WPAD, discussed in the next section, addresses this problem.
PAC, like preconfigured browsers, is used by some major ISPs today.
Web Proxy Autodiscovery Protocol
The Web Proxy Autodiscovery Protocol (WPAD) aims to provide a way for web
browsers to find and use nearby proxies, without requiring the end user to manually
Figure 20-10. Proxy auto-configuration
Browser
PAC server
Hi, I’ve been configured to
ask you for the PAC file.
Please send it to me.
HTTP/1.0 200 OK
Content-type: application/x-ns-proxy-autoconfig
Content-length: 176
function FindProxyForURL(url,host)
{
if (dnsDomain(host,".netscape.com")
return "DIRECT";
else
return "PROXY proxy1.joes-cache.com:8080; DIRECT";
}
Go to the proxy1 if it is available
or go directly to the origin server
if proxy1 is not reachable
Internet
Origin server
Proxy1
Requests to netscape.com domain are
sent directly to the server
Requests to all other domains are
sent to proxy1.cache1.com
Proxy Redirection Methods |465
configure a proxy setting and without relying on transparent traffic interception. The
general problem of defining a web proxy autodiscovery protocol is complicated by
the existence of many discovery protocols to choose from and the differences in
proxy-use configurations in different browsers.
This section contains an abbreviated and slightly reorganized version of the WPAD
Internet draft. The draft currently is being developed as part of the Web Intermediar-
ies Working Group of the IETF.
PAC file autodiscovery
WPAD enables HTTP clients to locate a PAC file and use the PAC file to discover the
name of an appropriate proxy server. WPAD does not directly determine the name of
the proxy server, because that would circumvent the additional capabilities provided
by PAC files (load balancing, request routing to an array of servers, automated
failover to backup proxy servers, and so on).
As shown in Figure 20-11, the WPAD protocol discovers a PAC file URL, also
known as a configuration URL (CURL). The PAC file executes a JavaScript program
that returns the address of an appropriate proxy server.
An HTTP client that implements the WPAD protocol:
• Uses WPAD to find the PAC file CURL
• Fetches the PAC file (a.k.a. configuration file, or CFILE) corresponding to the
CURL
• Executes the PAC file to determine the proxy server
• Sends HTTP requests to the proxy server returned by the PAC file
WPAD algorithm
WPAD uses a series of resource-discovery techniques to determine the proper PAC
file CURL. Multiple discovery techniques are specified, because not all organizations
Figure 20-11. WPAD determines the PAC URL, which determines the proxy server
PAC server Origin server
HTTP client
Internet
(b) Get PAC file
(c) Access server
through proxy
Proxy
(a) WPAD
discovery
466 |Chapter 20: Redirection and Load Balancing
can use all techniques. WPAD clients attempt each technique, one by one, until they
succeed in obtaining a CURL.
The current WPAD specification defines the following techniques, in order:
• DHCP (Dynamic Host Discovery Protocol)
• SLP (Service Location Protocol)
• DNS well-known hostnames
• DNS SRV records
• DNS service URLs in TXT records
Of these five mechanisms, only the DHCP and DNS well-known hostname tech-
niques are required for WPAD clients. We present more details in subsequent
sections.
The WPAD client sends a series of resource-discovery requests, using the discovery
mechanisms mentioned above, in order. Clients attempt only mechanisms that they
support. Whenever a discovery attempt succeeds, the client uses the information
obtained to construct a PAC CURL.
If a PAC file is retrieved successfully at that CURL, the process completes. If not, the
client resumes where it left off in the predefined series of resource-discovery requests.
If, after trying all discovery mechanisms, no PAC file is retrieved, the WPAD proto-
col fails and the client is configured to use no proxy server.
The client tries DHCP first, followed by SLP. If no PAC file is retrieved, the client
moves on to the DNS-based mechanisms.
The client cycles through the DNS SRV, well-known hostnames, and DNS TXT
record methods multiple times. Each time, the DNS query QNAME is made less and
less specific. In this manner, the client can locate the most specific configuration
information possible, but still can fall back on less specific information. Every DNS
lookup has the QNAME prefixed with “wpad” to indicate the resource type being
requested.
Consider a client with hostname johns-desktop.development.foo.com. This is the
sequence of discovery attempts a complete WPAD client would perform:
• DHCP
• SLP
• DNS A lookup on “QNAME=wpad.development.foo.com”
• DNS SRV lookup on “QNAME=wpad.development.foo.com”
• DNS TXT lookup on “QNAME=wpad.development.foo.com”
• DNS A lookup on “QNAME=wpad.foo.com”
• DNS SRV lookup on “QNAME=wpad.foo.com”
• DNS TXT lookup on “QNAME=wpad.foo.com”
Proxy Redirection Methods |467
Refer to the WPAD specification to get detailed pseudocode that addresses the entire
sequence of operations. The following sections discuss the two required mecha-
nisms, DHCP and DNS A lookup. For more details about the reminder of the CURL
discovery methods, refer to the WPAD specification.
CURL discovery using DHCP
For this mechanism to work, the CURLs must be stored on DHCP servers that
WPAD clients can query. The WPAD client obtains the CURL by sending a DHCP
query to a DHCP server. The CURL is contained in DHCP option code 252 (if the
DHCP server is configured with this information). All WPAD client implementa-
tions are required to support DHCP. The DHCP protocol is detailed in RFC 2131.
See RFC 2132 for a list of existing DHCP options.
If the WPAD client already has conducted DHCP queries during its initialization, the
DHCP server might already have supplied that value. If the value is not available
through a client OS API, the client sends a DHCPINFORM message to query the
DHCP server to obtain the value.
The DHCP option code 252 for WPAD is of type STRING and is of arbitrary size.
This string contains a URL that points to an appropriate PAC file. For example:
"http://server.domain/proxyconfig.pac"
DNS A record lookup
For this mechanism to work, the IP addresses of suitable proxy servers must be
stored on DNS servers that the WPAD clients can query. The WPAD client obtains
the CURL by sending an A record lookup to a DNS server. The result of a successful
lookup contains an IP address for an appropriate proxy server.
WPAD client implementations are required to support this mechanism. This should
be straightforward, as only basic DNS lookup of A records is required. See RFC 2219
for a description of using well-known DNS aliases for resource discovery. For WPAD,
the specification uses “well known alias” of “wpad” for web proxy autodiscovery.
The client performs the following DNS lookup:
QNAME=wpad.TGTDOM., QCLASS=IN, QTYPE=A
A successful lookup contains an IP address from which the WPAD client constructs
the CURL.
Retrieving the PAC file
Once a candidate CURL is created, the WPAD client usually makes a GET request
to the CURL. When making requests, WPAD clients are required to send Accept
headers with appropriate CFILE format information that they are capable of han-
dling. For example:
Accept: application/x-ns-proxy-autoconfig
468 |Chapter 20: Redirection and Load Balancing
In addition, if the CURL results in a redirect, the clients are required to follow the
redirect to its final destination.
When to execute WPAD
The web proxy autodiscovery process is required to occur at least as frequently as
one of the following:
• Upon startup of the web client—WPAD is performed only for the start of the
first instance. Subsequent instances inherit the settings.
• Whenever there is an indication from the networking stack that the IP address of
the client host has changed.
A web client can use either option, depending on what makes sense in its environ-
ment. In addition, the client must attempt a discovery cycle upon expiration of a pre-
viously downloaded PAC file in accordance with HTTP expiration. It’s important that
the client obey the timeouts and rerun the WPAD process when the PAC file expires.
Optionally, the client also may implement rerunning the WPAD process on failure of
the currently configured proxy if the PAC file does not provide an alternative.
Whenever the client decides to invalidate the current PAC file, it must rerun the
entire WPAD protocol to ensure it discovers the currently correct CURL. Specifi-
cally, there is no provision in the protocol to do an If-Modified-Since conditional
fetch of the PAC file.
A number of network round trips might be required during the WPAD protocol
broadcast and/or multicast communications. The WPAD protocol should not be
invoked at a more frequent rate than specified above (such as per-URL retrieval).
WPAD spoofing
The IE 5 implementation of WPAD enabled web clients to detect proxy settings
automatically, without user intervention. The algorithm used by WPAD prepends
the hostname “wpad” to the fully qualified domain name and progressively removes
subdomains until it either finds a WPAD server answering the hostname or reaches
the third-level domain. For instance, web clients in the domain a.b.microsoft.com
would query wpad.a.b.microsoft,wpad.b.microsoft.com, then wpad.microsoft.com.
This exposed a security hole, because in international usage (and certain other con-
figurations), the third-level domain may not be trusted. A malicious user could set up
a WPAD server and serve proxy configuration commands of her choice. Subsequent
versions of IE (5.01 and later) rectified the problem.
Timeouts
WPAD goes through multiple levels of discovery, and clients must make sure that
each phase is time-bound. When possible, limiting each phase to 10 seconds is
Cache Redirection Methods |469
considered reasonable, but implementors may choose a different value that is more
appropriate to their network properties. For example, a device implementation,
operating over a wireless network, might use a much larger timeout to account for
low bandwidth or high latency.
Administrator considerations
Administrators should configure at least one of the DHCP or DNS A record lookup
methods in their environments, as those are the only two that all compatible clients
are required to implement. Beyond that, configuring to support mechanisms earlier
in the search order will improve client startup time.
One of the major motivations for this protocol structure was to support client loca-
tion of nearby proxy servers. In many environments, there are several proxy servers
(workgroup, corporate gateway, ISP, backbone).
There are a number of possible points at which “nearness” decisions can be made in
the WPAD framework:
• DHCP servers for different subnets can return different answers. They also can
base decisions on the client cipaddr field or the client identifier option.
• DNS servers can be configured to return different SRV/A/TXT resource records
(RRs) for different domain suffixes (for example, QNAMEs wpad.marketing.big-
corp.com and wpad.development.bigcorp.com).
• The web server handling the CURL request can make decisions based on the
User-Agent header, Accept header, client IP address/subnet/hostname, topologi-
cal distribution of nearby proxy servers, etc. This can occur inside a CGI execut-
able created to handle the CURL. As mentioned earlier, it even can be a proxy
server handling the CURL requests and making these decisions.
• The PAC file may be expressive enough to select from a set of alternatives at
runtime on the client. CARP is based on this premise for an array of caches. It is
not inconceivable that the PAC file could compute some network distance or fit-
ness metrics to a set of candidate proxy servers and then select the “closest” or
“most responsive” server.
Cache Redirection Methods
We’ve discussed techniques to redirect traffic to general servers and specialized tech-
niques to vector traffic to proxies and gateways. This final section will explain some
of the more sophisticated redirection techniques used for caching proxy servers.
These techniques are more complex than the previously discussed protocols because
they try to be reliable, high-performance, and content-aware—dispatching requests
to locations likely to have particular pieces of content.
470 |Chapter 20: Redirection and Load Balancing
WCCP Redirection
Cisco Systems developed the Web Cache Coordination Protocol (WCCP) to enable
routers to redirect web traffic to proxy caches. WCCP governs communication
between routers and caches so that routers can verify caches (make sure they are up
and running), load balance among caches, and send specific types of traffic to specific
caches. WCCP Version 2 (WCCP2) is an open protocol. We’ll discuss WCCP2 here.
How WCCP redirection works
Here’s a brief overview of how WCCP redirection works for HTTP (WCCP redirects
other protocols similarly):
• Start with a network containing WCCP-enabled routers and caches that can
communicate with one another.
• A set of routers and their target caches form a WCCP service group. The config-
uration of the service group specifies what traffic is sent where, how traffic is
sent, and how load should be balanced among the caches in the service group.
• If the service group is configured to redirect HTTP traffic, routers in the service
group send HTTP requests to caches in the service group.
• When an HTTP request arrives at a router in the service group, the router chooses
one of the caches in the service group to serve the request (based on either a hash
on the request’s IP address or a mask/value set pairing scheme).
• The router sends the request packets to the cache, either by encapsulating the
packets with the cache’s IP address or by IP MAC forwarding.
• If the cache cannot serve the request, the packets are returned to the router for
normal forwarding.
• The members of the service group exchange heartbeat messages with one
another, continually verifying one another’s availability.
WCCP2 messages
There are four WCCP2 messages, described in Table 20-4.
Table 20-4. WCCP2 messages
Message name Who sends it Information carried
WCCP2_HERE_I_AM Cache to router These messages tell routers that caches are available to receive
traffic. The messages contain all of the cache’s service group
information. As soon as a cache joins a service group, it sends
these messages to all routers in the group. These messages
negotiate with routers sending WCCP2_I_SEE_YOU messages.
WCCP2_I_SEE_YOU Router to cache These messages respond to WCCP2_HERE_I_AM messages.
They are used to negotiate the packet forwarding method,
assignment method (who is the designated cache), packet
return method, and security.
Cache Redirection Methods |471
The WCCP2_HERE_I_AM message format is:
WCCP Message Header
Security Info Component
Service Info Component
Web-cache Identity Info Component
Web-cache View Info Component
Capability Info Component (optional)
Command Extension Component (optional)
The WCCP2_I_SEE_YOU message format is:
WCCP Message Header
Security Info Component
Service Info Component
Router Identity Info Component
Router View Info Component
Capability Info Component (optional)
Command Extension Component (optional)
The WCCP2_REDIRECT_ASSIGN message format is:
WCCP Message Header
Security Info Component
Service Info Component
Assignment Info Component, or Alternate Assignment Component
The WCCP2_REMOVAL_QUERY message format is:
WCCP Message Header
Security Info Component
Service Info Component
Router Query Info Component
Message components
Each WCCP2 message consists of a header and components. The WCCP header infor-
mation contains the message type (Here I Am, I See You, Assignment, or Removal
Query), WCCP version, and message length (not including the length of the header).
The components each begin with a four-octet header describing the component type
and length. The component length does not include the length of the component
header. The message components are described in Table 20-5.
WCCP2_REDIRECT_ASSIGN Designated cache to
router
These messages make assignments for load balancing; they
send bucket information for hash table load balancing or mask/
value set pair information for mask/value load balancing.
WCCP2_REMOVAL_QUERY Router to cache that has
not sent WCCP2_HERE_
I_AM messages for 2.5 ×
HERE_I_AM_T seconds
If a router does not receive WCCP2_HERE_I_AM messages reg-
ularly, the router sends this message to see if the cache should
be removed from the service group. The proper response from a
cache is three identical WCCP2_HERE_I_AM messages, sepa-
rated by HERE_I_AM_T/10 seconds.
Table 20-4. WCCP2 messages (continued)
Message name Who sends it Information carried
472 |Chapter 20: Redirection and Load Balancing
Service groups
Aservice group consists of a set of WCCP-enabled routers and caches that exchange
WCCP messages. The routers send web traffic to the caches in the service group. The
configuration of the service group determines how traffic is distributed to caches in
the service group. The routers and caches exchange service group configuration
information in Here I Am and I See You messages.
GRE packet encapsulation
Routers that support WCCP redirect HTTP packets to a particular server by encap-
sulating them with the server’s IP address. The packet encapsulation also contains an
IP header proto field that indicates Generic Router Encapsulation (GRE). The exist-
ence of the proto field tells the receiving proxy that it has an encapsulated packet.
Table 20-5. WCCP2 message components
Component Description
Security Info Contains the security option and security implementation. The security option can be:
WCCP2_NO_SECURITY (0)
WCCP2_MD5_SECURITY (1)
If the option is no security, the security implementation field does not exist. If the option is
MD5, the security implementation field is a 16-octet field containing the message check-
sum and Service Group password. The password can be no more than eight octets.
Service Info Describes the service group. The service type ID can have two values:
WCCP2_SERVICE_STANDARD (0)
WCCP2_SERVICE_DYNAMIC (1)
If the service type is standard, the service is a well-known service, defined entirely by ser-
vice ID. HTTP is an example of a well-known service. If the service type is dynamic, the fol-
lowing settings define the service: priority, protocol, service flags (which determine
hashing), and port.
Router Identity Info Contains the router IP address and ID, and lists (by IP address) all of the web caches with
which the router intends to communicate.
Web Cache Identity Info Contains the web cache IP address and redirection hash table mapping.
Router View Info Contains the router’s view of the service group (identities of the routers and caches).
Web Cache View Info Contains the web cache’s view of the service group.
Assignment Info Shows the assignment of a web cache to a particular hashing bucket.
Router Query Info Contains the router’s IP address, address of the web cache being queried, and ID of the last
router in the service group that received a Here I Am message from the web cache.
Capabilities Info Used byrouters toadvertise supportedpacket forwarding, load balancing,and packetreturn
methods; used by web caches to let routers know what method the web cache prefers.
Alternate Assignment Contains hash table assignment information for load balancing.
Assignment Map Contains mask/value set elements for service group.
Command Extension Used by web cachesto tell routers they areshutting down; used byrouters to acknowledge
a cache shutdown.
Internet Cache Protocol |473
Because the packet is encapsulated, the client IP address is not lost. Figure 20-12
illustrates GRE packet encapsulation.
WCCP load balancing
In addition to routing, WCCP routers can balance load among several receiving serv-
ers. WCCP routers and their receiving servers exchange heartbeat messages to let one
another know they are up and running. If a particular receiving server stops sending
heartbeat messages, the WCCP router sends request traffic directly to the Internet,
instead of redirecting it to that node. When the node returns to service, the WCCP
router begins receiving heartbeat messages again and resumes sending request traffic
to the node.
Internet Cache Protocol
The Internet Cache Protocol (ICP) allows caches to look for content hits in sibling
caches. If a cache does not have the content requested in an HTTP message, it can
find out if the content is in a nearby sibling cache and, if so, retrieve the content from
there, hopefully avoiding a more costly query to an origin server. ICP can be thought
of as a cache clustering protocol. It is a redirection protocol in the sense that the final
destination of an HTTP request message can be determined by a series of ICP queries.
ICP is an object discovery protocol. It asks nearby caches, all at the same time, if any
of them have a particular URL in their caches. The nearby caches send back a short
message saying “HIT” if they have that URL or “MISS” if they don’t. The cache is
then free to open an HTTP connection to a neighbor cache that has the object.
ICP is simple and lightweight. ICP messages are 32-bit packed structures in net-
work byte order, making them easy to parse. They are carried in UDP datagrams for
Figure 20-12. How a WCCP router changes an HTTP packet’s destination IP address
From:Client:
1.1.1.1
80 Passes through WCCP router
To:Joe’s Server:
2.2.2.2
80
From:Client:
1.1.1.1
80 To:Joe’s Server:
2.2.2.2
80
HTTP data
HTTP data
To Proxy:
3.3.3.3
8080
Proto:GRE
474 |Chapter 20: Redirection and Load Balancing
efficiency. UDP is an unreliable Internet protocol, which means that the data can get
destroyed in transit, so programs that speak ICP need to have timeouts to detect lost
datagrams.
Here is a brief description of the parts of an ICP message:
Opcode
The opcode is an 8-bit value that describes the meaning of the ICP message.
Basic opcodes are ICP_OP_QUERY request messages and ICP_OP_HIT and
ICP_OP_MISS response messages.
Version
The 8-bit version number describes the version number of the ICP protocol. The
version of ICP used by Squid, documented in Internet RFC 2186, is Version 2.
Message length
The total size in bytes of the ICP message. Because there are only 16 bits, the ICP
message size cannot be larger than 16,383 bytes. URLs usually are shorter than
16 KB; if they’re longer than that, many web applications will not process them.
Request number
ICP-enabled caches use the request number to keep track of multiple simulta-
neous requests and replies. An ICP reply message always must contain the same
request number as the ICP request message that triggered the reply.
Options
The 32-bit ICP options field is a bit vector containing flags that modify ICP
behavior. ICPv2 defines two flags, both of which modify ICP_OP_QUERY
requests. The ICP_FLAG_HIT_OBJ flag enables and disables the return of docu-
ment data in ICP responses. The ICP_FLAG_SRC_RTT flag requests an esti-
mate of the round-trip time to the origin server, as measured by a sibling cache.
Option data
The 32-bit option data is reserved for optional features. ICPv2 uses the low 16
bits of the option data to hold an optional round-trip time estimate from the sib-
ling to the origin server.
Sender host address
A historic field carrying the 32-bit IP address of the message sender; not used in
practice.
Payload
The contents of the payload vary depending on the message type. For ICP_OP_
QUERY, the payload is a 4-byte original requester host address followed by a
NUL-terminated URL. For ICP_OP_HIT_OBJ, the payload is a NUL-terminated
URL followed by a 16 bit object size, followed by the object data.
For more information about ICP, refer to informational RFCs 2186 and 2187. Excel-
lent ICP and peering references also are available from the U.S. National Laboratory
for Applied Network Research (http://www.nlanr.net/Squid/).
Cache Array Routing Protocol |475
Cache Array Routing Protocol
Proxy servers greatly reduce traffic to the Internet by intercepting requests from indi-
vidual users and serving cached copies of the requested web objects. However, as
the number of users grows, a high volume of traffic can overload the proxy servers
themselves.
One solution to this problem is to use multiple proxy servers to distribute the load to
a collection of servers. The Cache Array Routing Protocol (CARP) is a standard pro-
posed by Microsoft Corporation and Netscape Communication Corporation to
administer a collection of proxy servers such that an array of proxy servers appears to
clients as one logical cache.
CARP is an alternative to ICP. Both CARP and ICP allow administrators to improve
performance by using multiple proxy servers. This section discusses how CARP dif-
fers from ICP, the advantages and disadvantages of using CARP over ICP, and the
technical details of how the CARP protocol is implemented.
Upon a cache miss in ICP, the proxy server queries neighboring caches using an ICP
message format to determine the availability of the web object. The neighboring
caches respond with either a “HIT” or a “MISS,” and the requesting proxy server
uses these responses to select the most appropriate location from which to retrieve
the object. If the ICP proxy servers were arranged in a hierarchical fashion, a miss
would be elevated to the parent. Figure 20-13 diagrammatically shows how hits and
misses are resolved using ICP.
Figure 20-13. ICP queries
ParentBrowser Caching proxy
Client request
Sibling
Sibling
Poll (time n)
Poll (time n)
Hit or miss reply
(time n+1)
Hit or miss reply
(time n+1)
Request (time n+2)
Internet
Parent of proxy is polled if the
siblings return a MISS
476 |Chapter 20: Redirection and Load Balancing
Note that each of the proxy servers, connected together using the ICP protocol, is a
standalone cache server with redundant mirrors of content, meaning that duplicate
entries of web objects across proxy servers is possible. In contrast, the collection of
servers connected using CARP operates as a single, large server with each compo-
nent server containing only a fraction of the total cached documents. By applying a
hash function to the URL of a web object, CARP maps web objects to a specific
proxy server. Because each web object has a unique home, we can determine the
location of the object by a single lookup, rather than polling each of the proxy serv-
ers configured in the collection. Figure 20-14 summarizes the CARP approach.
Although Figure 20-14 shows the caching proxy as being the intermediary between
clients and proxy servers that distributes the load to the various proxy servers, it is
possible for this function to be served by the clients themselves. Commercial brows-
ers such as Internet Explorer and Netscape Navigator can be configured to compute
the hash function in the form of a plug-in that determines the proxy server to which
the request should be sent.
Deterministic resolution of the proxy server in CARP means that it isn’t necessary to
send queries to all the neighbors, which means that this method requires fewer inter-
cache messages to be sent out. As more proxy servers are added to the configura-
tion, the collective cache system will scale fairly well. However, a disadvantage of
CARP is that if one of the proxy servers becomes unavailable, the hash function
needs to be modified to reflect this change, and the contents of the proxy servers
must be reshuffled across the existing proxy servers. This can be expensive if the
proxy server crashes often. In contrast, redundant content in ICP proxy servers
Figure 20-14. CARP redirection
ParentBrowser Caching proxy
Client request
Sibling
Sibling
(Time n)
Response
(time n+1)
Request (time n+2)
Internet
Hash function used to decide
which sibling proxy cache
to contact
Cache Array Routing Protocol |477
means that reshuffling is not required. Another potential problem is that, because
CARP is a new protocol, existing proxy servers running only the ICP protocol may
not be included readily in a CARP collection.
Having described the difference between CARP and ICP, let us now describe CARP
in a little more detail. The CARP redirection method involves the following tasks:
• Keep a table of participating proxy servers. These proxy servers are polled peri-
odically to see which ones are still active.
• For each participating proxy server, compute a hash function. The value
returned by the hash function takes into account the amount of load this proxy
can handle.
• Define a separate hash function that returns a number based on the URL of the
requested web object.
• Take the sum of the hash function of the URL and the hash function of the
proxy servers to get an array of numbers. The maximum value of these numbers
determines the proxy server to use for the URL. Because the computed values are
deterministic, subsequent requests for the same web object will be forwarded to
the same proxy server.
These four chores can either be carried out on the browser, in a plug-in, or be com-
puted on an intermediate server.
For each collection of proxy servers, create a table listing all of the servers in the col-
lection. Each entry in the table should contain information about load factors, time-
to-live (TTL) countdown values, and global parameters such as how often members
should be polled. The load factor indicates how much load that machine can han-
dle, which depends on the CPU speed and hard drive capacity of that machine. The
table can be maintained remotely via an RPC interface. Once the fields in the tables
have been updated by RPC, they can be made available or published to downstream
clients and proxies. This publication is done in HTTP, allowing any client or proxy
server to consume the table information without introducing another inter-proxy
protocol. Clients and proxy servers simply use a well-known URL to retrieve the
table.
The hash function used must ensure that the web objects are statistically distributed
across the participating proxy servers. The load factor of the proxy server should be
used to determine the statistic probability of a web object being assigned to that
proxy.
In summary, the CARP protocol allows a group of proxy servers to be viewed as sin-
gle collective cache, instead of a group of cooperating but separate caches (as in ICP).
A deterministic request resolution path finds the home of a specific web object
within a single hop. This eliminates the inter-proxy traffic that often is generated to
478 |Chapter 20: Redirection and Load Balancing
find the web object in a group of proxy servers in ICP. CARP also avoids duplicate
copies of web objects being stored on different proxy servers, which has the advan-
tage that the cache system collectively has a larger capacity for storing web objects
but also has the disadvantage that a failure in any one proxy requires reshuffling
some of the cache contents to existing proxies.
Hyper Text Caching Protocol
Earlier, we discussed ICP, a protocol that allows proxy caches to query siblings
about the presence of documents. ICP, however, was designed with HTTP/0.9 in
mind and therefore allows caches to send just the URL when querying a sibling
about the presence of a resource. Versions 1.0 and 1.1 of HTTP introduced many
new request headers that, along with the URL, are used to make decisions about
document matching, so simply sending the URL in a request may not result in accu-
rate responses.
The Hyper Text Caching Protocol (HTCP) reduces the probability of false hits by
allowing siblings to query each other for the presence of documents using the URL
and all of the request and response headers. Further, HTCP allows sibling caches to
monitor and request the addition and deletion of selected documents in each
other’s caches and to make changes in the caching policies of each other’s cached
documents.
Figure 20-13, which illustrates an ICP transaction, also can be used to illustrate an
HTCP transaction—HTCP is just another object discovery protocol. If a nearby
cache has the document, the requesting cache can open an HTTP connection to the
cache to get a copy of the document. The difference between an ICP and an HTCP
transaction is in the level of detail in the requests and responses.
The structure of HTCP messages is illustrated in Figure 20-15. The Header portion
includes the message length and message versions. The Data portion starts with the
data length and includes opcodes, response codes, and some flags and IDs, and it ter-
minates with the actual data. An optional Authentication section may follow the
Data section.
Details of the message fields are as follows:
Header
The Header section consists of a 32-bit message length, an 8-bit major protocol
version, and an 8-bit minor protocol version. The message length includes all of
the header, data, and authentication sizes.
Data
The Data section contains the HTCP message and has the structure illustrated in
Figure 20-15. The data components are described in Table 20-6.
Hyper Text Caching Protocol |479
Figure 20-15. HTCP message format
Table 20-6. HTCP data components
Component Description
Data length A 16-bit value of the number of bytes in the Data section including the length of the Length field
itself.
Opcode The 4-bit operation code for the HTCP transaction. The full list of opcodes is provided in
Table 20-7.
Response code A 4-bit key indicating the success or failure of the transaction. The possible values are:
•0—Authentication was not used, but is needed
•1—Authentication was used, but is not satisfactory
•2—Unimplemented opcode
•3—Major version not supported
•4—Minor version not supported
•5—Inappropriate, disallowed, or undesirable opcode
F1 F1 is overloaded—if the message is a request, F1is a 1-bit flag set by the requestor indicating that it
needs a response (F1=1); if the message is a response, F1 is a 1-bit flag indicating whether the
response is to be interpreted as a response to the overall message (F1=1) or just as a response to
the Opcode data fields (F1=0).
RR A 1-bit flag indicating that the message is a request (RR=0) or a response (RR=1).
Transaction ID A 32-bit value that, combined with the requestor’s network address, uniquely identifies the HTCP
transaction.
Opcode data Opcode data is opcode-dependent. See Table 20-7.
Transaction ID
Opcode data
Opcode
Major version
Message length
031
Signature
Key name
Sig expire
Sig time
Auth length
Key name
Sig expire
Sig time
Minor version Data length
Response code Reserved F1 RR
480 |Chapter 20: Redirection and Load Balancing
Table 20-7 lists the HTCP opcodes and their corresponding data types.
HTCP Authentication
The authentication portion of the HTCP message is optional. Its structure is illus-
trated in Figure 20-15, and its components are described inTable 20-8.
Setting Caching Policies
The SET message allows caches to request changes in the caching policies of cached
documents. The headers that can be used in SET messages are described in Table 20-9.
Table 20-7. HTCP opcodes
Opcode Value Description Response codes Opcode data
NOP 0 Essentially a “ping” operation. Always 0 None
TST 1 0 if entity is present, 1 if
entity is not present
Contains the URL and
request headers in the
request and just
response headers in
the response
MON 2 0 if accepted, 1 if refused
SET 3 The SET message allows caches to
request changes in caching policies.
See Table 20-9 for a list of the headers
that can be used in SET messages.
0 if accepted, 1 if ignored
CLR 4 0 if I had it, but it’s now gone;
1 if I had it, but I am keeping
it; and 2 if I didn’t have it
Table 20-8. HTCP authentication components
Component Description
Auth length The 16-bit number of bytes in the Authentication section of the message, including the length of
the Length field itself.
Sig time A 32-bit number representing the number of seconds since 00:00:00 Jan 1, 1970 GMT at the time
that the signature is generated.
Sig expire A 32-bit number representing the number of seconds since 00:00:00 Jan 1, 1970 GMT when the sig-
nature will expire.
Key name A string thatspecifies the nameof the sharedsecret. The Keysection has twoparts: the 16-bitlength
in bytes of the string that follows, followed by the stream of uninterrupted bytes of the string.
Signature The HMAC-MD5 digest with a B value of 64 (representing the source and destination IP addresses
and ports), the major and minor HTCP versions of the message, the Sig time and Sig expires values,
the full HTCP data, and the key. The Signature also has two parts: the 16-bit length in bytes of the
string, followed by the string.
For More Information |481
By allowing request and response headers to be sent in query messages to sibling
caches, HTCP can decrease the false-hit rate in cache queries. By further allowing
sibling caches to exchange policy information with each other, HTCP can improve
sibling caches’ ability to cooperate with each other.
For More Information
For more information, consult the following references:
DNS and Bind
Cricket Liu, Paul Albitz, and Mike Loukides, O’Reilly & Associates, Inc.
http://www.wrec.org/Drafts/draft-cooper-webi-wpad-00.txt
“Web Proxy Auto-Discovery Protocol.”
http://home.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html
“Navigator Proxy Auto-Config File Format.”
http://www.ietf.org/rfc/rfc2186.txt
IETF RFC 2186, “Intercache Communication Protocol (ICP) Version 2,” by D.
Wessels and K. Claffy.
http://icp.ircache.net/carp.txt
“Cache Array Routing Protocol v1.0.”
http://www.ietf.org/rfc/rfc2756.txt
IETF RFC 2756, “Hyper Text Caching Protocol (HTCP/0.0),” by P. Vixie and D.
Wessels.
Table 20-9. List of Cache headers for modifying caching policies
Header Description
Cache-Vary The requestor has learned that the content varies on a set of headers different from the set in the
response Vary header. This header overrides the response Vary header.
Cache-Location The list of proxy caches that also may have copies of this object.
Cache-Policy The requestor has learned the caching policies for this object in more detail than is specified in the
response headers. Possible values are: “no-cache,” meaning that the response is not cacheable but
may be shareable among simultaneous requestors; “no-share,” meaning that the object is not
shareable; and “no-cache-cookie,” meaning that the content may change as a result of cookies and
caching therefore is not advised.
Cache-Flags The requestor has modified the object’s caching policies and the object may have to be treated spe-
cially and not necessarily in accordance with the object’s actual policies.
Cache-Expiry The actual expiration time for the document as learned by the requestor.
Cache-MD5 The requestor-computed MD5 checksum of the object, which may be different from the value in the
Content-MD5 header, or may be supplied because the object does not have a Content-MD5 header.
Cache-to-Origin The requestor-measured round-trip time to an origin server. The format of the values in this header
is <origin server name or ip> <average round-trip time in seconds> <number of samples> <num-
ber of router hops between requestor and origin server>.
482 |Chapter 20: Redirection and Load Balancing
http://www.ietf.org/internet-drafts/draft-wilson-wrec-wccp-v2-00.txt
draft-wilson-wrec-wccp-v2-01.txt, “Web Cache Communication Protocol V2.0,”
by M. Cieslak, D. Forster, G. Tiwana, and R. Wilson.
http://www.ietf.org/rfc/rfc2131.txt?number=2131
“Dynamic Host Configuration Protocol.”
http://www.ietf.org/rfc/rfc2132.txt?number=2132
“DHCP Options and BOOTP Vendor Extensions.”
http://www.ietf.org/rfc/rfc2608.txt?number=2608
“Service Location Protocol, Version 2.”
http://www.ietf.org/rfc/rfc2219.txt?number=2219
“Use of DNS Aliases for Network Services.”
483
CHAPTER 21
Logging and Usage Tracking
Almost all servers and proxies log summaries of the HTTP transactions they process.
This is done for a variety of reasons: usage tracking, security, billing, error detection,
and so on. In this chapter, we take a brief tour of logging, examining what informa-
tion about HTTP transactions typically is logged and what some of the common log
formats contain.
What to Log?
For the most part, logging is done for two reasons: to look for problems on the server
or proxy (e.g., which requests are failing), and to generate statistics about how web
sites are accessed. Statistics are useful for marketing, billing, and capacity planning
(for instance, determining the need for additional servers or bandwidth).
You could log all of the headers in an HTTP transaction, but for servers and proxies
that process millions of transactions per day, the sheer bulk of all of that data quickly
would get out of hand. You also would end up logging a lot of information that you
don’t really care about and may never even look at.
Typically, just the basics of a transaction are logged. A few examples of commonly
logged fields are:
• HTTP method
• HTTP version of client and server
• URL of the requested resource
• HTTP status code of the response
• Size of the request and response messages (including any entity bodies)
• Timestamp of when the transaction occurred
• Referer and User-Agent header values
484 |Chapter 21: Logging and Usage Tracking
The HTTP method and URL tell what the request was trying to do—for example,
GETting a resource or POSTing an order form. The URL can be used to track popu-
larity of pages on the web site.
The version strings give hints about the client and server, which are useful in debug-
ging strange or unexpected interactions between clients and servers. For example, if
requests are failing at a higher-than-expected rate, the version information may point
to a new release of a browser that is unable to interact with the server.
The HTTP status code tells what happened to the request: whether it was success-
ful, the authorization attempt failed, the resource was found, etc. (See “Status
Codes” in Chapter 3 for a list of HTTP status codes.)
The size of the request/response and the timestamp are used mainly for accounting
purposes; i.e., to track how many bytes flowed into, out of, or through the applica-
tion. The timestamp also can be used to correlate observed problems with the
requests that were being made at the time.
Log Formats
Several log formats have become standard, and we’ll discuss some of the most com-
mon formats in this section. Most commercial and open source HTTP applications
support logging in one or more of these common formats. Many of these applica-
tions also support the ability of administrators to configure log formats and create
their own custom formats.
One of the main benefits of supporting (for applications) and using (for administra-
tors) these more standard formats rests in the ability to leverage the tools that have
been built to process and generate basic statistics from these logs. Many open source
and commercial packages exist to crunch logs for reporting purposes, and by utilizing
standard formats, applications and their administrators can plug into these resources.
Common Log Format
One of the most common log formats in use today is called, appropriately, the
Common Log Format. Originally defined by NCSA, many servers use this log for-
mat as a default. Most commercial and open source servers can be configured to use
this format, and many commercial and freeware tools exist to help parse common
log files. Table 21-1 lists, in order, the fields of the Common Log Format.
Table 21-1. Common Log Format fields
Field Description
remotehost The hostname or IP address of the requestor’s machine (IP if the server was not configured to perform
reverse DNS or cannot look up the requestor’s hostname)
username If an ident lookup was performed, the requestor’s authenticated usernamea
Log Formats |485
Example 21-1 lists a few examples of Common Log Format entries.
In these examples, the fields are assigned as follows:
Note that the remotehost field can be either a hostname, as in http-guide.com,oran
IP address, such as 209.1.32.44.
The dashes in the second (username) and third (auth-username) fields indicate that
the fields are empty. This indicates that either an ident lookup did not occur (second
field empty) or authentication was not performed (third field empty).
Combined Log Format
Another commonly used log format is the Combined Log Format. This format is
supported by servers such as Apache. The Combined Log Format is very similar to
the Common Log Format; in fact, it mirrors it exactly, with the addition of two fields
(listed in Table 21-2). The User-Agent field is useful in noting which HTTP client
applications are making the logged requests, while the Referer field provides more
detail about where the requestor found this URL.
auth-username If authentication was performed, the username with which the requestor authenticated
timestamp The date and time of the request
request-line The exact text of the HTTP request line, “GET /index.html HTTP/1.1”
response-code The HTTP status code that was returned in the response
response-size The Content-Length of the response entity—if no entity was returned in the response, a zero is logged
aRFC 931 describes the ident lookup used in this authentication. The ident protocol was discussed in Chapter 5.
Example 21-1. Common Log Format
209.1.32.44 - - [03/Oct/1999:14:16:00 -0400] "GET / HTTP/1.0" 200 1024
http-guide.com - dg [03/Oct/1999:14:16:32 -0400] "GET / HTTP/1.0" 200 477
http-guide.com - dg [03/Oct/1999:14:16:32 -0400] "GET /foo HTTP/1.0" 404 0
Field Entry 1 Entry 2 Entry 2
remotehost 209.1.32.44 http-guide.com http-guide.com
username <empty> <empty> <empty>
auth-username <empty> dg dg
timestamp 03/Oct/1999:14:16:00 -0400 03/Oct/1999:14:16:32 -0400 03/Oct/1999:14:16:32 -0400
request-line GET / HTTP/1.0 GET / HTTP/1.0 GET /foo HTTP/1.0
response-code 200 200 404
response-size 1024 477 0
Table 21-1. Common Log Format fields (continued)
Field Description
486 |Chapter 21: Logging and Usage Tracking
Example 21-2 gives an example of a Combined Log Format entry.
In Example 21-2, the Referer and User-Agent fields are assigned as follows:
The first seven fields of the example Combined Log Format entry in Example 21-2
are exactly as they would be in the Common Log Format (see the first entry in
Example 21-1). The two new fields, Referer and User-Agent, are tacked onto the end
of the log entry.
Netscape Extended Log Format
When Netscape entered into the commercial HTTP application space, it defined for
its servers many log formats that have been adopted by other HTTP application
developers. Netscape’s formats derive from the NCSA Common Log Format, but
they extend that format to incorporate fields relevant to HTTP applications such as
proxies and web caches.
The first seven fields in the Netscape Extended Log Format are identical to those in
the Common Log Format (see Table 21-1). Table 21-3 lists, in order, the new fields
that the Netscape Extended Log Format introduces.
Table 21-2. Additional Combined Log Format fields
Field Description
Referer The contents of the Referer HTTP header
User-Agent The contents of the User-Agent HTTP header
Example 21-2. Combined Log Format
209.1.32.44 - - [03/Oct/1999:14:16:00 -0400] "GET / HTTP/1.0" 200 1024 "http://www.joes-
hardware.com/" "5.0: Mozilla/4.0 (compatible; MSIE 5.0; Windows 98)"
Field Value
Referer http://www.joes-hardware.com/
User-Agent 5.0: Mozilla/4.0 (compatible; MSIE 5.0; Windows 98)
Table 21-3. Additional Netscape Extended Log Format fields
Field Description
proxy-response-code If the transaction went through a proxy, the HTTP response code from the server to the proxy
proxy-response-size If the transaction went through a proxy, the Content-Length of the server’s response entity sent
to the proxy
client-request-size The Content-Length of any body or entity in the client’s request to the proxy
proxy-request-size If the transaction went through a proxy, the Content-Length of any body or entity in the proxy’s
request to the server
client-request-hdr-size The length, in bytes, of the client’s request headers
Log Formats |487
Example 21-3 gives an example of a Netscape Extended Log Format entry.
In this example, the extended fields are assigned as follows:
The first seven fields of the example Netscape Extended Log Format entry in
Example 21-3 mirror the entries in the Common Log Format example (see the first
entry in Example 21-1).
Netscape Extended 2 Log Format
Another Netscape log format, the Netscape Extended 2 Log Format, takes the
Extended Log Format and adds further information relevant to HTTP proxy and web
caching applications. These extra fields help paint a better picture of the interactions
between an HTTP client and an HTTP proxy application.
The Netscape Extended 2 Log Format derives from the Netscape Extended Log For-
mat, and its initial fields are identical to those listed in Table 21-3 (it also extends the
Common Log Format fields listed in Table 21-1).
proxy-response-hdr-size If the transaction went through a proxy, the length, in bytes, of the proxy’s response headers
that were sent to the requestor
proxy-request-hdr-size If the transaction went through a proxy, the length, in bytes, of the proxy’s request headers
that were sent to the server
server-response-hdr-size The length, in bytes, of the server’s response headers
proxy-timestamp If the transaction went through a proxy, the elapsed time for the request and response to travel
through the proxy, in seconds
Example 21-3. Netscape Extended Log Format
209.1.32.44 - - [03/Oct/1999:14:16:00-0400] "GET / HTTP/1.0" 200 1024 200 1024 0 0 215 260
279 254 3
Field Value
proxy-response-code 200
proxy-response-size 1024
client-request-size 0
proxy-request-size 0
client-request-hdr-size 215
proxy-response-hdr-size 260
proxy-request-hdr-size 279
server-response-hdr-size 254
proxy-timestamp 3
Table 21-3. Additional Netscape Extended Log Format fields (continued)
Field Description
488 |Chapter 21: Logging and Usage Tracking
Table 21-4 lists, in order, the additional fields of the Netscape Extended 2 Log Format.
Example 21-4 gives an example of a Netscape Extended 2 Log Format entry.
The extended fields in this example are assigned as follows:
The first 16 fields in the Netscape Extended 2 Log Format entry in Example 21-4 mir-
ror the entries in the Netscape Extended Log Format example (see Example 21-3).
Table 21-5 lists the valid Netscape route codes.
Table 21-6 lists the valid Netscape finish codes.
Table 21-4. Additional Netscape Extended 2 Log Format fields
Field Description
route The route that the proxy used to make the request for the client (see Table 21-5)
client-finish-status-code The client finish status code; specifies whether the client request to the proxy completed suc-
cessfully (FIN) or was interrupted (INTR)
proxy-finish-status-code The proxy finish status code; specifies whether the proxy request to the server completed suc-
cessfully (FIN) or was interrupted (INTR)
cache-result-code The cache result code; tells how the cache responded to the requesta
aTable 21-7 lists the Netscape cache result codes.
Example 21-4. Netscape Extended 2 Log Format
209.1.32.44 - - [03/Oct/1999:14:16:00-0400] "GET / HTTP/1.0" 200 1024 200 1024 0 0 215 260
279 254 3 DIRECT FIN FIN WRITTEN
Field Value
route DIRECT
client-finish-status-code FIN
proxy-finish-status-code FIN
cache-result-code WRITTEN
Table 21-5. Netscape route codes
Value Description
DIRECT The resource was fetched directly from the server.
PROXY(host:port) The resource was fetched through the proxy “host.”
SOCKS(socks:port) The resource was fetched through the SOCKS server “host.”
Table 21-6. Netscape finish status codes
Value Description
- The request never even started.
FIN The request was completed successfully.
Log Formats |489
Table 21-7 lists the valid Netscape cache codes.*
Netscape applications, like many other HTTP applications, have other log formats
too, including a Flexible Log Format and a means for administrators to output cus-
tom log fields. These formats allow administrators greater control and the ability to
customize their logs by choosing which parts of the HTTP transaction (headers, sta-
tus, sizes, etc.) to report in their logs.
The ability for administrators to configure custom formats was added because it is
difficult to predict what information administrators will be interested in getting
from their logs. Many other proxies and servers also have the ability to emit custom
logs.
Squid Proxy Log Format
The Squid proxy cache (http://www.squid-cache.org) is a venerable part of the Web. Its
roots trace back to one of the early web proxy cache projects (ftp://ftp.cs.colorado.edu/
pub/techreports/schwartz/Harvest.Conf.ps.Z). Squid is an open source project that has
been extended and enhanced by the open source community over the years. Many
tools have been written to help administer the Squid application, including tools to
help process, audit, and mine its logs. Many subsequent proxy caches adopted the
Squid format for their own logs so that they could leverage these tools.
INTR The request was interrupted by the client or ended by a proxy/server.
TIMEOUT The request was timed out by the proxy/server.
* Chapter 7 discusses HTTP caching in detail.
Table 21-7. Netscape cache codes
Code Description
- The resource was uncacheable.
WRITTEN The resource was written into the cache.
REFRESHED The resource was cached and it was refreshed.
NO-CHECK The cached resource was returned; no freshness check was done.
UP-TO-DATE The cached resource was returned; a freshness check was done.
HOST-NOT-AVAILABLE The cached resource was returned; no freshness check was done because the remote server was
not available.
CL-MISMATCH The resource was not written to the cache; the write was aborted because the Content-Length
did not match the resource size.
ERROR The resource was not written to the cache due to some error; for example, a timeout occurred or
the client aborted the transaction.
Table 21-6. Netscape finish status codes (continued)
Value Description
490 |Chapter 21: Logging and Usage Tracking
The format of a Squid log entry is fairly simple. Its fields are summarized in
Table 21-8.
Example 21-5 gives an example of a Squid Log Format entry.
The fields are assigned as follows:
Table 21-8. Squid Log Format fields
Field Description
timestamp The timestamp when the request arrived, in seconds since January 1, 1970 GMT.
time-elapsed The elapsed time for request and response to travel through the proxy, in milliseconds.
host-ip The IP address of the client’s (requestor’s) host machine.
result-code/status The result field is a Squid-ism that tells what action the proxy took during this requesta; the
code field is the HTTP response code that the proxy sent to the client.
aTable 21-9 lists the various result codes and their meanings.
size The length of the proxy’s response to the client, including HTTP response headers and body,
in bytes.
method The HTTP method of the client’s request.
url The URL in the client’s request.b
bRecall from Chapter 2 that proxies often log the entire requested URL, so if a username and password component are in the URL, a proxy
can inadvertently record this information.
rfc931-identc
cThe rfc931-ident, hierarchy/from, and content-type fields were added in Squid 1.1. Previous versions did not have these fields.
The client’s authenticated username.d
dRFC 931 describes the ident lookup used in this authentication.
hierarchy/from Like the route field in Netscape formats, the hierarchy field tells what route the proxy used to
make the request for the client.e The from field tells the name of the server that the proxy
used to make the request.
ehttp://squid.nlanr.net/Doc/FAQ/FAQ-6.html#ss6.6 lists all of the valid Squid hierarchy codes.
content-type The Content-Type of the proxy response entity.
Example 21-5. Squid Log Format
99823414 3001 209.1.32.44 TCP_MISS/200 4087 GET http://www.joes-hardware.com - DIRECT/
proxy.com text/html
Field Value
timestamp 99823414
time-elapsed 3001
host-ip 209.1.32.44
action-code TCP_MISS
status 200
size 4087
method GET
URL http://www.joes-hardware.com
Log Formats |491
Table 21-9 lists the various Squid result codes.*
RFC 931 ident -
hierarchy DIRECTa
from proxy.com
content-type text/html
aThe DIRECT Squid hierarchy value is the same as the DIRECT route value in Netscape log formats.
* Several of these action codes deal more with the internals of the Squid proxy cache, so not all of them are
used by other proxies that implement the Squid Log Format.
Table 21-9. Squid result codes
Action Description
TCP_HIT A valid copy of the resource was served out of the cache.
TCP_MISS The resource was not in the cache.
TCP_REFRESH_HIT The resource was in the cache but needed to be checked for freshness. The proxy revalidated
the resource with the server and found that the in-cache copy was indeed still fresh.
TCP_REF_FAIL_HIT The resource was in the cache but needed to be checked for freshness. However, the revalida-
tion failed (perhaps the proxy could not connect to the server), so the “stale” resource was
returned.
TCP_REFRESH_MISS The resource was in the cache but needed to be checked for freshness. Upon checking with
the server, the proxy learned that the resource in the cache was out of date and received a
new version.
TCP_CLIENT_REFRESH_MISS The requestor sent a Pragma: no-cache or similar Cache-Control directive, so the proxy was
forced to fetch the resource.
TCP_IMS_HIT The requestor issued a conditional request, which was validated against the cached copy of
the resource.
TCP_SWAPFAIL_MISS The proxy thought the resource was in the cache but for some reason could not access it.
TCP_NEGATIVE_HIT A cached response was returned, but the response was a negatively cached response. Squid
supports the notion of caching errors for resources—for example, caching a 404 Not Found
response—so if multiple requests go through the proxy-cache for an invalid resource, the
error is served from the proxy cache.
TCP_MEM_HIT A valid copy of the resource was served out of the cache, and the resource was in the proxy
cache’s memory (as opposed to having to access the disk to retrieve the cached resource).
TCP_DENIED The request for this resource was denied, probably because the requestor does not have per-
mission to make requests for this resource.
TCP_OFFLINE_HIT The requested resource was retrieved from the cache during its offline mode. Resources are
not validated when Squid (or another proxy using this format) is in offline mode.
UDP_* The UDP_* codes indicate that requests were received through the UDP interface to the
proxy. HTTP normally uses the TCP transport protocol, so these requests are not using the
HTTP protocol.a
Field Value
492 |Chapter 21: Logging and Usage Tracking
Hit Metering
Origin servers often keep detailed logs for billing purposes. Content providers need
to know how often URLs are accessed, advertisers want to know how often their ads
are shown, and web authors want to know how popular their content is. Logging
works well for tracking these things when clients visit web servers directly.
However, caches stand between clients and servers and prevent many accesses from
reaching servers (the very purpose of caches).*Because caches handle many HTTP
requests and satisfy them without visiting the origin server, the server has no record
that a client accessed its content, creating omissions in log files.
Missing log data makes content providers resort to cache busting for their most impor-
tant pages. Cache busting refers to a content producer intentionally making certain
content uncacheable, so all requests for this content must go to the origin server.†
This allows the origin server to log the access. Defeating caching might yield better
logs, but it slows down requests and increases load on the origin server and network.
Because proxy caches (and some clients) keep their own logs, if servers could get
access to these logs—or at least have a crude way to determine how often their con-
tent is served by a proxy cache—cache busting could be avoided. The proposed Hit
Metering protocol, an extension to HTTP, suggests a solution to this problem. The
Hit Metering protocol requires caches to periodically report cache access statistics to
origin servers.
UDP_HIT A valid copy of the resource was served out of the cache.
UDP_MISS The resource was not in the cache.
UDP_DENIED The request for this resource was denied, probably because the requestor does not have per-
mission to make requests for this resource.
UDP_INVALID The request that the proxy received was invalid.
UDP_MISS_NOFETCH Used by Squid during specific operation modes or in the cache of frequent failures. A cache
miss was returned and the resource was not fetched.
NONE Logged sometimes with errors.
TCP_CLIENT_REFRESH See TCP_CLIENT_REFRESH_MISS.
TCP_SWAPFAIL See TCP_SWAPFAIL_MISS.
UDP_RELOADING See UDP_MISS_NOFETCH.
aSquid has its own protocol for making these requests: ICP. This protocol is used for cache-to-cache requests. See http://www.squid-cache.org
for more information.
* Recall that virtually every browser has a cache.
† Chapter 7 describes how HTTP responses can be marked as uncacheable.
Table 21-9. Squid result codes (continued)
Action Description
Hit Metering |493
RFC 2227 defines the Hit Metering protocol in detail. This section provides a brief
tour of the proposal.
Overview
The Hit Metering protocol defines an extension to HTTP that provides a few basic
facilities that caches and servers can implement to share access information and to
regulate how many times cached resources can be used.
Hit Metering is, by design, not a complete solution to the problem caches pose for
logging access, but it does provide a basic means for obtaining metrics that servers
want to track. The Hit Metering protocol has not been widely implemented or
deployed (and may never be). That said, a cooperative scheme like Hit Metering
holds some promise of providing accurate access statistics while retaining caching
performance gains. Hopefully, that will be motivation to implement the Hit Meter-
ing protocol instead of marking content uncacheable.
The Meter Header
The Hit Metering extension proposes the addition of a new header, Meter, that
caches and servers can use to pass to each other directives about usage and report-
ing, much like the Cache-Control header allows caching directives to be exchanged.
Table 21-10 defines the various directives and who can pass them in the Meter header.
Table 21-10. Hit Metering directives
Directive Abbreviation Who Description
will-report-and-limit w Cache The cache is capable of reporting usage and obeying any usage limits
the server specifies.
wont-report x Cache The cache is able to obey usage limits but won’t report usage.
wont-limit y Cache The cache is able to report usage but won’t limit usage.
count c Cache The reporting directive, specified as “uses/reuses” integers—for
example, “:count=2/4”.a
aHit Metering defines a use as satisfying a request with the response, whereas a reuse is revalidating a client request.
max-uses u Server Allows the server to specify the maximum number of times a response
can be used by a cache—for example, “max-uses=100”.
max-reuses r Server Allows the server to specify the maximum number of times a response
can be reused by a cache—for example, “max-reuses=100”.
do-report d Server The server requires proxies to send usage reports.
dont-report e Server The server does not want usage reports.
timeout t Server Allows the server to specify a timeout on the metering of a resource.
The cache should send a report at or before the specified timeout, plus
or minus 1 minute. The timeout is specified in minutes—for example,
“timeout=60”.
wont-ask n Server The server does not want any metering information.
494 |Chapter 21: Logging and Usage Tracking
Figure 21-1 shows an example of Hit Metering in action. The first part of the transac-
tion is just a normal HTTP transaction between a client and proxy cache, but in the
proxy request, note the insertion of the Meter header and the response from the
server. Here, the proxy is informing the server that it is capable of doing Hit Meter-
ing, and the server in turn is asking the proxy to report its hit counts.
The request completes as it normally would, from the client’s perspective, and the
proxy begins tracking hits to that resource on behalf of the server. Later, the proxy
tries to revalidate the resource with the server. The proxy embeds the metered infor-
mation it has been tracking in the conditional request to the server.
Figure 21-1. Hit Metering example
Request message
Client
GET http://joes-hardware.com/ HTTP/1.1
Host: www.joes-hardware.com
Accept: *
www.joes-hardware.com
GET / HTTP/1.1
Host: www.joes-hardware.com
Meter: will-report-and-limit
Connection: Meter
Response message
HTTP/1.1 200 OK
Date: Fri, 06 Dec 1996 18:44:29 GMT
Content-length: 3152
Content-type: text/html
Connection: Meter
ETag: "v1.27"
Meter: do-report
[...]
Proxy
Client
Later, the cache revalidates the
response and at the same time
reports the hit count
www.joes-hardware.com
GET / HTTP/1.1
Host: www.joes-hardware.com
Meter: 12/4
If-None-Match: "v1.27"
Connection: Meter
HTTP/1.1 304 Not Modified
[...]
Response sent to client, cached, and
used for subsequent requests
Proxy
Proxy
HTTP/1.1 200 OK
Date: Fri, 06 Dec 1996 18:44:29 GMT
Content-length: 3152
Content-type: text/html
[...]
For More Information |495
A Word on Privacy
Because logging really is an administrative function that servers and proxies per-
form, the whole operation is transparent to users. Often, they may not even be aware
that their HTTP transactions are being logged—in fact, many users probably do not
even know that they are using the HTTP protocol when accessing content on the
Web.
Web application developers and administrators need to be aware of the implications
of tracking a user’s HTTP transactions. Much can be gleaned about a user based on
the information he retrieves. This information obviously can be put to bad use—
discrimination, harassment, blackmail, etc. Web servers and proxies that log must be
vigilant in protecting the privacy of their end users.
Sometimes, such as in work environments, tracking a user’s usage to make sure he is
not goofing off may be appropriate, but administrators also should make public the
fact that people’s transactions are being monitored.
In short, logging is a very useful tool for the administrator and developer—just be
aware of the privacy infringements that logs can have without the permission or
knowledge of the users whose actions are being logged.
For More Information
For more information on logging, refer to:
http://httpd.apache.org/docs/logs.html
“Apache HTTP Server: Log Files.” Apache HTTP Server Project web site.
http://www.squid-cache.org/Doc/FAQ/FAQ-6.html
“Squid Log Files.” Squid Proxy Cache web site.
http://www.w3.org/Daemon/User/Config/Logging.html#common-logfile-format
“Logging Control in W3C httpd.”
http://www.w3.org/TR/WD-logfile.html
“Extended Log File Format.”
http://www.ietf.org/rfc/rfc2227.txt
RFC 2227, “Simple Hit-Metering and Usage-Limiting for HTTP,” by J. Mogul
and P. Leach.
PART VI
Appendixes
This collection of appendixes contains useful reference tables, background informa-
tion, and tutorials on a variety of topics relevant to HTTP architecture and imple-
mentation:
• Appendix A, URI Schemes
• Appendix B, HTTP Status Codes
• Appendix C, HTTP Header Reference
• Appendix D, MIME Types
• Appendix E, Base-64 Encoding
• Appendix F, Digest Authentication
• Appendix G, Language Tags
• Appendix H, MIME Charset Registry
499
APPENDIX A
URI Schemes
Many URI schemes have been defined, but few are in common use. Generally speak-
ing, those URI schemes with associated RFCs are in more common use, though there
are a few schemes that have been developed by leading software corporations (nota-
bly Netscape and Microsoft), but not formalized, that also are in wide use.
The W3C maintains a list of URI schemes, which you can view at:
http://www.w3.org/Addressing/schemes.html
The IANA also maintains a list of URL schemes, at:
http://www.iana.org/assignments/uri-schemes
Table A-1 informally describes some of the schemes that have been proposed and
those that are in active use. Note that many of the approximately 90 schemes in the
table are not widely used, and many are extinct.
Table A-1. URI schemes from the W3C registry
Scheme Description RFCs
about Netscape scheme to explore aspects of the browser. For example: about by itself is the same as
choosing “About Communicator” from the Navigator Help menu, about:cache displays disk-
cache statistics, and about:plugins displays information about configured plug-ins. Other
browsers, such as Microsoft Internet Explorer, also use this scheme.
acap Application Configuration Access Protocol. 2244
afp For file-sharing services using the Apple Filing Protocol (AFP) protocol, defined as part of the
expired IETF draft-ietf-svrloc-afp-service-01.txt.
afs Reserved for future use by the Andrew File System.
callto Initiates a Microsoft NetMeeting conference session, such as:
callto: ws3.joes-hardware.com/joe@joes-hardware.com
chttp The CHTTP caching protocol defined by Real Networks. RealPlayer does not cache all items
streamed by HTTP. Instead, you designate files to cache by using chttp:// instead of http:// in
the file’s URL. When RealPlayer reads a CHTTP URL in a SMIL file, it first checks its disk cache for
the file. If the file isn’t present, it requests the file through HTTP, storing the file in its cache.
500 |Appendix A: URI Schemes
cid The use of [MIME] within email to convey web pages and their associated images requires a
URL scheme to permit the HTML to refer to the images or other data included in the message.
The Content-ID URL, “cid:”, serves that purpose.
2392
2111
clsid Allows Microsoft OLE/COM (Component Object Model) classes to be referenced. Used to insert
active objects into web pages.
data Allows inclusion of small, constant data items as “immediate” data. This URL encodes the text/
plain string “A brief note”:
data:A%20brief%20note
2397
date Proposal for scheme to support dates, as in date:1999-03-04T20:42:08.
dav To ensure correct interoperation based on this specification, the IANA must reserve the URI
namespaces starting with “DAV:” and with “opaquelocktoken:” for use by this specification, its
revisions, and related WebDAV specifications.
2518
dns Used by REBOL software.
See http //www.rebol.com/users/valurl.html.
eid The external ID (eid) scheme provides a mechanism by which the local application can refer-
ence data that has been obtained by other, non-URL scheme means. The scheme is intended to
provide a general escape mechanism to allow access to information for applications that are too
specialized to justify their own schemes. There is some controversy about this URI.
See http //www.ics.uci.edu/pub/ietf/uri/draft-finseth-url-00.txt.
fax The “fax” scheme describes a connection to a terminal that can handle telefaxes (facsimile
machines).
2806
file Designates files accessible on a particular host computer. A hostname can be included, but the
scheme is unusual in that it does not specify an Internet protocol or access method for such
files; as such, its utility in network protocols between hosts is limited.
1738
finger The finger URL has the form:
finger://host[:port][/<request>]
The <request> must conform with the RFC 1288 request format.
See http //www.ics.uci.edu/pub/ietf/uri/draft-ietf-uri-url-finger-03.txt.
freenet URIs for information in the Freenet distributed information system.
See http //freenet.sourceforge.net.
ftp File Transfer Protocol scheme. 1738
gopher The archaic gopher protocol. 1738
gsm-sms URIs for the GSM mobile phone short message service.
h323, h324 Multimedia conferencing URI schemes.
See http //www.ics.uci.edu/pub/ietf/uri/draft-cordell-sg16-conv-url-00.txt.
hdl The Handle System is a comprehensive system for assigning, managing, and resolving persis-
tent identifiers, known as “handles,” for digital objects and other resources on the Internet.
Handles can be used as URNs.
See http //www.handle.net.
hnews HNEWS is an HTTP-tunneling variant of the NNTP news protocol. The syntax of hnews URLs is
designed to be compatible with the current common usage of the news URL scheme.
See http //www.ics.uci.edu/pub/ietf/uri/draft-stockwell-hnews-url-00.txt.
Table A-1. URI schemes from the W3C registry (continued)
Scheme Description RFCs
URI Schemes |501
http The HTTP protocol. Read this book for more information. 2616
https HTTP over SSL.
See http //sitesearch.netscape.com/eng/ssl3/draft302.txt.
iioploc CORBA extensions. The Interoperable Name Service defines one URL-format object reference,
iioploc, that can be typed into a program to reach defined services at remote locations, includ-
ing the Naming Service. For example, this iioploc identifier:
iioploc //www.omg.org/NameService
would resolve to the CORBA Naming Service running on the machine whose IP address corre-
sponded to the domain name www.omg.org.
See http //www.omg.org.
ilu The Inter-Language Unification (ILU) system is a multilingual object interface system. The
object interfaces provided by ILU hide implementation distinctions between different lan-
guages, different address spaces, and different operating system types. ILU can be used to build
multilingual object-oriented libraries (“class libraries”) with well-specified,
language-independent interfaces. It also can be used to implement distributed systems.
See ftp://parcftp.parc.xerox.com/pub/ilu/ilu.html.
imap The IMAP URL scheme is used to designate IMAP servers, mailboxes, messages, MIME bodies
[MIME], and search programs on Internet hosts accessible using the IMAP protocol.
2192
IOR CORBA interoperable object reference.
See http //www.omg.org.
irc The irc URL scheme is used to refer to either Internet Relay Chat (IRC) servers or individual enti-
ties (channels or people) on IRC servers.
See http //www.w3.org/Addressing/draft-mirashi-url-irc-01.txt.
isbn Proposed scheme for ISBN book references.
See http //lists.w3.org/Archives/Public/www-talk/1991NovDec/0008.html.
java Identifies Java classes.
javascript The Netscape browser processes javascript URLs, evaluates the expression after the colon (:), if
there is one, and loads a page containing the string value of the expression, unless it is
undefined.
jdbc Used in the Java SQL API.
ldap Allows Internet clients direct access to the LDAP protocol. 2255
lid The Local Identifier (lid:) scheme.
See draft-blackketter-lid-00.
lifn A Location-Independent File Name (LIFN) for the Bulk File Distribution distributed storage sys-
tem developed at UTK.
livescript Old name for JavaScript.
lrq See h323.
mailto The mailto URL scheme is used to designate the Internet mailing address of an individual or
service.
2368
mailserver Old proposal from 1994–1995 to let an entire message be encoded in a URL, so that (for exam-
ple) the URL can automatically send email to a mail server for subscribing to a mailing list.
Table A-1. URI schemes from the W3C registry (continued)
Scheme Description RFCs
502 |Appendix A: URI Schemes
md5 MD5 is a cryptographic checksum.
mid The mid scheme uses (a part of) the message-id of an email message to refer to a specific
message.
2392
2111
mocha See javascript.
modem The modem scheme describes a connection to a terminal that can handle incoming data calls. 2806
mms, mmst,
mmsu
Scheme for Microsoft Media Server (MMS) to stream Active Streaming Format (ASF) files. To
force UDP transport, use the mmsu scheme. To force TCP transport, use mmst.
news The news URL scheme is used to refer to either news groups or individual articles of USENET
news. A news URL takes one of two forms: news:<newsgroup-name> or news:<message-id>.
1738
1036
nfs Used to refer to files and directories on NFS servers. 2224
nntp An alternative method of referencing news articles, useful for specifying news articles from
NNTP servers. An nntp URL looks like:
nntp //<host>:<port>/<newsgroup-name>/<article-num>
Note that while nntp URLs specify a unique location for the article resource, most NNTP servers
currently on the Internet are configured to allow access only from local clients, and thus nntp
URLs do not designate globally accessible resources. Hence, the news form of URL is preferred
as a way of identifying news articles.
1738
977
opaquelocktoken A WebDAV lock token, represented as a URI, that identifies a particular lock. A lock token is
returned by every successful LOCK operation in the lockdiscovery property in the response body
and also can be found through lock discovery on a resource. See RFC 2518.
path The path scheme defines a uniformly hierarchical namespace where a path URN is a sequence
of components and an optional opaque string.
See http //www.hypernews.org/~liberte/www/path html.
phone Used in “URLs for Telephony”; replaced with tel: in RFC 2806.
pop The POP URL designates a POP email server, and optionally a port number, authentication
mechanism, authentication ID, and/or authorization ID.
2384
pnm Real Networks’s streaming protocol.
pop3 The POP3 URL scheme allows a URL to specify a POP3 server, allowing other protocols to use a
general “URL to be used for mail access” in place of an explicit reference to POP3. Defined in
expired draft-earhart-url-pop3-00.txt.
printer Abstract URLs for use with the Service Location standard.
See draft-ietf-srvloc-printer-scheme-02.txt.
prospero Names resources to be accessed via the Prospero Directory Service. 1738
res Microsoft scheme that specifies a resource to be obtained from a module. Consists of a string or
numerical resource type, and a string or numerical ID.
rtsp Real-time streaming protocol that is the basis for Real Networks’s modern streaming control
protocols.
2326
rvp URLs for the RVP rendezvous protocol, used to notify the arrival of users on a computer
network.
See draft-calsyn-rvp-01.
Table A-1. URI schemes from the W3C registry (continued)
Scheme Description RFCs
URI Schemes |503
rwhois RWhois is an Internet directory access protocol, defined in RFC 1714 and RFC 2167. The RWhois
URL gives clients direct access to rwhois.
See http //www.rwhois.net/rwhois/docs/.
rx An architecture to allow remote graphical applications to display data inside web pages.
See http //www.w3.org/People/danield/papers/mobgui/.
sdp Session Description Protocol (SDP) URLs. See RFC 2327.
service The service scheme is used to provide access information for arbitrary network services. These
URLs provide an extensible framework for client-based network software to obtain configura-
tion information required to make use of network services.
2609
sip The sip* family of schemes are used to establish multimedia conferences using the Session Ini-
tiation Protocol (SIP).
2543
shttp S-HTTP is a superset of HTTP designed to secure HTTP connections and provide a wide variety of
mechanisms to provide for confidentiality, authentication, and integrity. It has not been widely
deployed, and it has mostly been supplanted with HTTPS SSL-encrypted HTTP.
See http //www.homeport.org/~adam/shttp.html.
snews SSL-encrypted news.
STANF Old proposal for stable network filenames. Related to URNs.
See http //web3.w3.org/Addressing/#STANF.
t120 See h323.
tel URL to place a call using the telephone network. 2806
telephone Used in previous drafts of tel.
telnet Designates interactive services that may be accessed by the Telnet protocol. A telnet URL takes
the form:
telnet //<user>:<password>@<host>:<port>/
1738
tip Supports TIP atomic Internet transactions. 2371
2372
tn3270 Reserved, as per ftp //ftp.isi.edu/in-notes/iana/assignments/url-schemes.
tv The TV URL names a particular television broadcast channel. 2838
uuid Universally unique identifiers (UUIDs) contain no information about location. They also are
known as globally unique identifiers (GUIDs). They are persistent over time, like URNs, and con-
sist of a 128-bit unique ID. UUID URIs are useful in situations where a unique identifier is
required that cannot or should not be tied to a particular physical root namespace (such as a
DNS name).
See draft-kindel-uuid-uri-00.txt.
urn Persistent, location-independent, URNs. 2141
vemmi Allows versatile multimedia interface (VEMMI) client software and VEMMI terminals to connect
to VEMMI-compliant services. VEMMI is an international standard for online multimedia
services.
2122
videotex Allows videotex client software or terminals to connect to videotex services compliant with the
ITU-T and ETSI videotex standards.
See http //www.ics.uci.edu/pub/ietf/uri/draft-mavrakis-videotex-url-spec-01.txt.
Table A-1. URI schemes from the W3C registry (continued)
Scheme Description RFCs
504 |Appendix A: URI Schemes
view-source Netscape Navigator source viewers. These view-source URLs display HTML that was generated
with JavaScript.
wais The wide area information service—an early form of search engine. 1738
whois++ URLs for the WHOIS++ simple Internet directory protocol.
See http //martinh.net/wip/whois-url.txt.
1835
whodp The Widely Hosted Object Data Protocol (WhoDP) exists to communicate the current location
and state of large numbers of dynamic, relocatable objects. A WhoDP program “subscribes” to
locate and receive information about an object and “publishes” to control the location and visi-
ble state of an object.
See draft-mohr-whodp-00.txt.
z39.50r, z39.50s Z39.50 session and retrieval URLs. Z39.50 is an information retrieval protocol that does not fit
neatly into a retrieval model designed primarily around the stateless fetch of data. Instead, it
models a general user inquiry as a session-oriented, multi-step task, any step of which may be
suspended temporarily while the server requests additional parameters from the client before
continuing.
2056
Table A-1. URI schemes from the W3C registry (continued)
Scheme Description RFCs
505
APPENDIX B
HTTP Status Codes
This appendix is a quick reference of HTTP status codes and their meanings.
Status Code Classifications
HTTP status codes are segmented into five classes, shown in Table B-1.
Status Codes
Table B-2 is a quick reference for all the status codes defined in the HTTP/1.1 speci-
fication, providing a brief summary of each. “Status Codes” in Chapter 3 goes into
more detailed descriptions of these status codes and their uses.
Table B-1. Status code classifications
Overall range Defined range Category
100–199 100–101 Informational
200–299 200–206 Successful
300–399 300–305 Redirection
400–499 400–415 Client error
500–599 500–505 Server error
Table B-2. Status codes
Status code Reason phrase Meaning
100 Continue An initial part of the request was received, and the client should
continue.
101 Switching Protocols The server is changing protocols, as specified by the client, to one
listed in the Upgrade header.
200 OK The request is okay.
201 Created The resource was created (for requests that create server objects).
506 |Appendix B: HTTP Status Codes
202 Accepted The request was accepted, but the server has not yet performed any
action with it.
203 Non-Authoritative Information The transaction was okay, except the information contained in the
entity headers was not from the origin server, but from a copy of the
resource.
204 No Content The response message contains headers and a status line, but no
entity body.
205 Reset Content Another code primarily for browsers; basically means that the
browser should clear any HTML form elements on the current page.
206 Partial Content A partial request was successful.
300 Multiple Choices A client has requested a URL that actually refers to multiple
resources. This code is returned along with a list of options; the user
can then select which one he wants.
301 Moved Permanently The requested URL has been moved. The response should contain a
Location URL indicating where the resource now resides.
302 Found Like the 301 status code, but the move is temporary. The client
should use the URL given in the Location header to locate the
resource temporarily.
303 See Other Tells the client that the resource should be fetched using a different
URL. This new URL is in the Location header of the response message.
304 Not Modified Clients can make their requests conditional by the request headers
they include. This code indicates that the resource has not changed.
305 Use Proxy The resource must be accessed through a proxy, the location of the
proxy is given in the Location header.
306 (Unused) This status code currently is not used.
307 Temporary Redirect Like the 301 status code; however, the client should use the URL
given in the Location header to locate the resource temporarily.
400 Bad Request Tells the client that it sent a malformed request.
401 Unauthorized Returned along with appropriate headers that ask the client to
authenticate itself before it can gain access to the resource.
402 Payment Required Currently this status code is not used, but it has been set aside for
future use.
403 Forbidden The request was refused by the server.
404 Not Found The server cannot find the requested URL.
405 Method Not Allowed A request was made with a method that is not supported for the
requested URL. The Allow header should be included in the
response to tell the client what methods are allowed on the
requested resource.
406 Not Acceptable Clients can specify parameters about whattypes of entities they are
willing to accept. This code is used when the server has no resource
matching the URL that is acceptable for the client.
407 Proxy Authentication Required Like the 401 status code, but used for proxy servers that require
authentication for a resource.
Table B-2. Status codes (continued)
Status code Reason phrase Meaning
Status Codes |507
408 Request Timeout If a client takes too long to complete its request, a server can send
back this status code and close down the connection.
409 Conflict The request is causing some conflict on a resource.
410 Gone Like the 404 status code, except that the server once held the
resource.
411 Length Required Servers use this codewhen they require a Content-Length header in
the request message. The server will not accept requests for the
resource without the Content-Length header.
412 Precondition Failed If a client makes a conditional request and one of the conditions
fails, this response code is returned.
413 Request Entity Too Large The client sent an entity body that is larger than the server can or
wants to process.
414 Request URI Too Long The client sent a request witha request URL thatis larger thanwhat
the server can or wants to process.
415 Unsupported Media Type The client sent an entity of a content type that the server does not
understand or support.
416 Requested Range Not Satisfiable The request message requested a range of a given resource, and
that range either was invalid or could not be met.
417 Expectation Failed The request contained an expectation in the Expect request header
that could not be satisfied by the server.
500 Internal Server Error The server encounteredan error thatprevented itfrom servicingthe
request.
501 Not Implemented The client made a request that is beyond the server’s capabilities.
502 Bad Gateway Aserver acting asa proxy or gatewayencountered a bogusresponse
from the next link in the request response chain.
503 Service Unavailable The server cannot currently service the requestbut will be able to in
the future.
504 Gateway Timeout Similar to the 408 status code, except that the response is coming
from a gateway or proxy that has timed out waiting for a response
to its request from another server.
505 HTTP Version Not Supported The serverreceived a requestin a versionof the protocol thatit can’t
or won’t support.
Table B-2. Status codes (continued)
Status code Reason phrase Meaning
508
APPENDIX C
HTTP Header Reference
It’s almost amusing to remember that the first version of HTTP, 0.9, had no headers.
While this certainly had its down sides, its fun to marvel in its simplistic elegance.
Well, back to reality. Today there are a horde of HTTP headers, many part of the
specification and still others that are extensions to it. This appendix provides some
background on these official and extension headers. It also acts as an index for the
various headers in this book, pointing out where their concepts and features are dis-
cussed in the running text. Most of these headers are simple up-front; it’s the interac-
tions with each other and other features of HTTP where things get hairy. This
appendix provides a bit of background for the headers listed and directs you to the
sections of the book where they are discussed at length.
The headers listed in this appendix are drawn from the HTTP specifications, related
documents, and our own experience poking around with HTTP messages and the
various servers and clients on the Internet.
This list is far from exhaustive. There are many other extension headers floating
around on the Web, not to mention those potentially used in private intranets.
Nonetheless, we have attempted to make this list as complete as possible. See RFC
2616 for the current version of the HTTP/1.1 specification and a list of official head-
ers and their specification descriptions.
Accept
The Accept header is used by clients to let servers know what media types are acceptable.
The value of the Accept header field is a list of media types that the client can use. For
instance, your web browser cannot display every type of multimedia object on the Web. By
including an Accept header in your requests, your browser can save you from downloading
a video or other type of object that you can’t use.
The Accept header field also may include a list of quality values (q values) that tell the
server which media type is preferred, in case the server has multiple versions of the media
type. See Chapter 17 for a complete discussion of content negotiation and q values.
Accept-Encoding |509
Type
Request header
Notes
“*” is a special value that is used to wildcard media types. For example,
“*/*” represents all types, and “image/*” represents all image types.
Examples
Accept: text/*, image/*
Accept: text/*, image/gif, image/jpeg;q=1
Accept-Charset
The Accept-Charset header is used by clients to tell servers what character sets are accept-
able or preferred. The value of this request header is a list of character sets and possibly
quality values for the listed character sets. The quality values let the server know which
character set is preferred, in case the server has the document in multiple acceptable char-
acter sets. See Chapter 17 for a complete discussion of content negotiation and q values.
Type
Request header
Notes
As with the Accept header, “*” is a special character. If present, it repre-
sents all character sets, except those that also are mentioned explicitly in
the value. If it’s not present, any charset not in the value field has a
default q value of zero, with the exception of the iso-latin-1 charset,
which gets a default of 1.
Basic Syntax
Accept-Charset: 1# ((charset | "*") [";" "q" "=" qvalue])
Example
Accept-Charset: iso-latin-1
Accept-Encoding
The Accept-Encoding header is used by clients to tell servers what encodings are accept-
able. If the content the server is holding is encoded (perhaps compressed), this request
header lets the server know whether the client will accept it. Chapter 17 contains a
complete description of the Accept-Encoding header.
Type
Request header
Basic Syntax
Accept-Encoding: 1# ((content-coding | "*") [";" "q" "=" qvalue])
Examples
*
Accept-Encoding:
Accept-Encoding: gzip
Accept-Encoding: compress;q=0.5, gzip;q=1
* The empty Accept-Encoding example is not a typo. It refers to the identity encoding—that is, the unencoded
content. If the Accept-Encoding header is present and empty, only the unencoded content is acceptable.
510 |Appendix C: HTTP Header Reference
Accept-Language
The Accept-Language request header functions like the other Accept headers, allowing
clients to inform the server about what languages (e.g., the natural language for content)
are acceptable or preferred. Chapter 17 contains a complete description of the Accept-
Language header.
Type
Request header
Basic Syntax
Accept-Language: 1# (language-range [";" "q" "=" qvalue])
language-range = ((1*8ALPHA * ("-" 1*8ALPHA)) | "*")
Examples
Accept-Language: en
Accept-Language: en;q=0.7, en-gb;q=0.5
Accept-Ranges
The Accept-Ranges header differs from the other Accept headers—it is a response header
used by servers to tell clients whether they accept requests for ranges of a resource. The
value of this header tells what type of ranges, if any, the server accepts for a given resource.
A client can attempt to make a range request on a resource without having received this
header. If the server does not support range requests for that resource, it can respond with
an appropriate status code*and the Accept-Ranges value “none”. Servers might want to
send the “none” value for normal requests to discourage clients from making range
requests in the future.
Chapter 17 contains a complete description of the Accept-Ranges header.
Type
Response header
Basic Syntax
Accept-Ranges: 1# range-unit | none
Examples
Accept-Ranges: none
Accept-Ranges: bytes
Age
The Age header tells the receiver how old a response is. It is the sender’s best guess as to how
long ago the response was generated by or revalidated with the origin server. The value of the
header is the sender’s guess, a delta in seconds. See Chapter 7 for more on the Age header.
Type
Response header
Notes
HTTP/1.1 caches must include an Age header in every response they send.
* For example, status code 416 (see “400–499: Client Error Status Codes” in Chapter 3).
Cache-Control |511
Basic Syntax
Age: delta-seconds
Example
Age: 60
Allow
The Allow header is used to inform clients what HTTP methods are supported on a partic-
ular resource.
Type
Response header
Notes
An HTTP/1.1 server sending a 405 Method Not Allowed response must
include an Allow header.*
Basic Syntax
Allow: #Method
Example
Allow: GET, HEAD
Authorization
The Authorization header is sent by a client to authenticate itself with a server. A client will
include this header in its request after receiving a 401 Authentication Required response
from a server. The value of this header depends on the authentication scheme in use. See
Chapter 14 for a detailed discussion of the Authorization header.
Type
Response header
Basic Syntax
Authorization: authentication-scheme #authentication-param
Example
Authorization: Basic YnJpYW4tdG90dHk6T3ch
Cache-Control
The Cache-Control header is used to pass information about how an object can be cached.
This header is one of the more complex headers introduced in HTTP/1.1. Its value is a
caching directive, giving caches special instructions about an object’s cacheability.
In Chapter 7, we discuss caching in general as well as the specific details about this header.
Type
General header
Example
Cache-Control: no-cache
* See “Status Codes” in Chapter 3 for more on the 405 status code.
512 |Appendix C: HTTP Header Reference
Client-ip
The Client-ip header is an extension header used by some older clients and some proxies to
transmit the IP address of the machine on which the client is running.
Type
Extension request header
Notes
Implementors should be aware that the information provided in the
value of this header is not secure.
Basic Syntax
Client-ip: ip-address
Example
Client-ip: 209.1.33.49
Connection
The Connection header is a somewhat overloaded header that can lead to a bit of confu-
sion. This header was used in HTTP/1.0 clients that were extended with keep-alive
connections for control information.*In HTTP/1.1, the older semantics are mostly recog-
nized, but the header has taken on a new function.
In HTTP/1.1, the Connection header’s value is a list of tokens that correspond to header
names. Applications receiving an HTTP/1.1 message with a Connection header are
supposed to parse the list and remove any of the headers in the message that are in the
Connection header list. This is mainly for proxies, allowing a server or other proxy to
specify hop-by-hop headers that should not be passed along.
One special token value is “close”. This token means that the connection is going to be
closed after the response is completed. HTTP/1.1 applications that do not support persis-
tent connections need to insert the Connection header with the “close” token in all
requests and responses.
Type
General header
Notes
While RFC 2616 does not specifically mention keep-alive as a connec-
tion token, some browsers (including those sending HTTP/1.1 as their
versions) use it in making requests.
Basic Syntax
Connection: 1# (connection-token)
Examples
Connection: close
* See Chapter 4 for more on keep-alive and persistent connections.
Content-Language |513
Content-Base
The Content-Base header provides a way for a server to specify a base URL for resolving
URLs found in the entity body of a response.*The value of the Content-Base header is an
absolute URL that can be used to resolve relative URLs found inside the entity.
Type
Entity header
Notes
This header is not defined in RFC 2616; it was previously defined in RFC
2068, an earlier draft of the HTTP/1.1 specification, and has since been
removed from the official specification.
Basic Syntax
Content-Base: absoluteURL
Example
Content-Base: http://www.joes-hardware.com/
Content-Encoding
The Content-Encoding header is used to specify whether any encodings have been
performed on the object. By encoding the content, a server can compress it before sending
the response. The value of the Content-Encoding header tells the client what type or types
of encoding have been performed on the object. With that information, the client can then
decode the message.
Sometimes more than one encoding is applied to an entity, in which case the encodings
must be listed in the order in which they were performed.
Type
Entity header
Basic Syntax
Content-Encoding: 1# content-coding
Examples
Content-Encoding: gzip
Content-Encoding: compress, gzip
Content-Language
The Content-Language header tells the client the natural language that should be under-
stood in order to understand the object. For instance, a document written in French would
have a Content-Language value indicating French. If this header is not present in the
response, the object is intended for all audiences. Multiple languages in the header’s value
indicate that the object is suitable for audiences of each language listed.
One caveat about this header is that the header’s value may just represent the natural
language of the intended audience of this object, not all or any of the languages contained
* See Chapter 2 for more on base URLs.
514 |Appendix C: HTTP Header Reference
in the object. Also, this header is not limited to text or written data objects; images, video,
and other media types can be tagged with their intended audiences’ natural languages.
Type
Entity header
Basic Syntax
Content-Language: 1# language-tag
Examples
Content-Language: en
Content-Language: en, fr
Content-Length
The Content-Length header gives the length or size of the entity body. If the header is in a
response message to a HEAD HTTP request, the value of the header indicates the size that
the entity body would have been had it been sent.
Type
Entity header
Basic Syntax
Content-Length: 1*DIGIT
Example
Content-Length: 2417
Content-Location
The Content-Location header is included in an HTTP message to give the URL corre-
sponding to the entity in the message. For objects that may have multiple URLs, a response
message can include a Content-Location header indicating the URL of the object used to
generate the response. The Content-Location can be different from the requested URL.
This generally is used by servers that are directing or redirecting a client to a new URL.
If the URL is relative, it should be interpreted relative to the Content-Base header. If the
Content-Base header is not present, the URL used in the request should be used.
Type
Entity header
Basic Syntax
Content-Location: (absoluteURL | relativeURL)
Example
Content-Location: http://www.joes-hardware.com/index.html
Content-MD5
The Content-MD5 header is used by servers to provide a message-integrity check for the
message body. Only an origin server or requesting client should insert a Content-MD5
Content-Type |515
header in the message. The value of the header is an MD5 digest*of the (potentially
encoded) message body.
The value of this header allows for an end-to-end check on the data, useful for detecting
unintentional modifications to the data in transit. It is not intended to be used for security
purposes.
RFC 1864 defines this header in more detail.
Type
Entity header
Notes
The MD5 digest value is a base-64 (see Appendix E) or 128-bit MD5
digest, as defined in RFC 1864.
Basic Syntax
Content-MD5: md5-digest
Example
Content-MD5: Q2h1Y2sgSW51ZwDIAXR5IQ==
Content-Range
The Content-Range header is sent as the result of a request that transmitted a range of a
document. It provides the location (range) within the original entity that this entity repre-
sents. It also gives the length of the entire entity.
If an “*” is present in the value instead of the length of the entire entity, this means that the
length was not known when the response was sent.
See Chapter 15 for more on the Content-Range header.
Type
Entity header
Notes
Servers responding with the 206 Partial Content response code must not
include a Content-Range header with an “*” as the length.
Example
Content-Range: bytes 500-999 / 5400
Content-Type
The Content-Type header tells the media type of the object in the message.
Type
Entity header
Basic Syntax
Content-Type: media-type
Example
Content-Type: text/html; charset=iso-latin-1
* The MD5 digest is defined in RFC 1864.
516 |Appendix C: HTTP Header Reference
Cookie
The Cookie header is an extension header used for client identification and tracking.
Chapter 11 talks about the Cookie header and its use in detail (also see “Set-Cookie”).
Type
Extension request header
Example
Cookie: ink=IUOK164y59BC708378908CFF89OE5573998A115
Cookie2
The Cookie2 header is an extension header used for client identification and tracking.
Cookie2 is used to identify what version of cookies a requestor understands. It is defined in
greater detail in RFC 2965.
Chapter 11 talks about the Cookie2 header and its use in detail.
Type
Extension request header
Example
Cookie2: $version="1"
Date
The Date header gives the date and time at which the message was created. This header is
required in servers’ responses because the time and date at which the server believes the
message was created can be used by caches in evaluating the freshness of a response. For
clients, this header is completely optional, although it’s good form to include it.
Type
General header
Basic Syntax
Date: HTTP-date
Examples
Date: Tue, 3 Oct 1997 02:15:31 GMT
HTTP has a few specific date formats. This one is defined in RFC 822
and is the preferred format for HTTP/1.1 messages. However, in earlier
specifications of HTTP, the date format was not spelled out as well, so
server and client implementors have used other formats, which need to
be supported for the sake of legacy. You will run into date formats like
the one specified in RFC 850, as well as dates in the format produced by
the asctime( ) system call. Here they are for the date represented above:
Date: Tuesday, 03-Oct-97 02:15:31 GMT RFC 850 format
Date: Tue Oct 3 02:15:31 1997 asctime( ) format
The asctime( ) format is looked down on because it is in local time and it
does not specify its time zone (e.g., GMT). In general, the date header
should be in GMT; however, robust applications should handle dates
that either do not specify the time zone or include Date values in non-
GMT time.
From |517
ETag
The ETag header provides the entity tag for the entity contained in the message. An entity
tag is basically a way of identifying a resource.
Entity tags and their relationship to resources are discussed in detail in Chapter 15.
Type
Entity header
Basic Syntax
ETag: entity-tag
Examples
ETag: "11e92a-457b-31345aa"
ETag: W/"11e92a-457b-3134b5aa"
Expect
The Expect header is used by clients to let servers know that they expect certain behavior.
This header currently is closely tied to the response code 100 Continue (see “100–199:
Informational Status Codes” in Chapter 3).
If a server does not understand the Expect header’s value, it should respond with a status
code of 417 Expectation Failed.
Type
Request header
Basic Syntax
Expect: 1# ("100-continue" | expectation-extension)
Example
Expect: 100-continue
Expires
The Expires header gives a date and time at which the response is no longer valid. This
allows clients such as your browser to cache a copy and not have to ask the server if it is
still valid until after this time has expired.
Chapter 7 discusses how the Expires header is used—in particular, how it relates to caches
and having to revalidate responses with the origin server.
Type
Entity header
Basic Syntax
Expires: HTTP-date
Example
Expires: Thu, 03 Oct 1997 17:15:00 GMT
From
The From header says who the request is coming from. The format is just a valid Internet
email address (specified in RFC 1123) for the user of the client.
518 |Appendix C: HTTP Header Reference
There are potential privacy issues with using/populating this header. Client implementors
should be careful to inform their users and give them a choice before including this header
in a request message. Given the potential for abuse by people collecting email addresses for
unsolicited mail messages, woe to the implementor who broadcasts this header unan-
nounced and has to answer to angry users.
Type
Request header
Basic Syntax
From: mailbox
Example
From: slurp@inktomi.com
Host
The Host header is used by clients to provide the server with the Internet hostname and
port number of the machine from which the client wants to make a request. The hostname
and port are those from the URL the client was requesting.
The Host header allows servers to differentiate different relative URLs based on the host-
name, giving the server the ability to host several different hostnames on the same machine
(i.e., the same IP address).
Type
Request header
Notes
HTTP/1.1 clients must include a Host header in all requests. All HTTP/
1.1 servers must respond with the 400 Bad Request status code to
HTTP/1.1 clients that do not provide a Host header.
Basic Syntax
Host: host [":" port]
Example
Host: www.hotbot.com:80
If-Modified-Since
The If-Modified-Since request header is used to make conditional requests. A client can use
the GET method to request a resource from a server, having the response hinge on whether
the resource has been modified since the client last requested it.
If the object has not been modified, the server will respond with a 304 Not Modified
response, instead of with the resource. If the object has been modified, the server will
respond as if it was a non-conditional GET request. Chapter 7 discusses conditional
requests in detail.
Type
Request header
Basic Syntax
If-Modified-Since: HTTP-date
Example
If-Modified-Since: Thu, 03 Oct 1997 17:15:00 GMT
If-Range |519
If-Match
Like the If-Modified-Since header, the If-Match header can be used to make a request
conditional. Instead of a date, the If-Match request uses an entity tag. The server compares
the entity tag in the If-Match header with the current entity tag of the resource and returns
the object if the tags match.
The server should use the If-Match value of “*” to match any entity tag it has for a resource;
“*” will always match, unless the server no longer has the resource.
This header is useful for updating resources that a client or cache already has. The resource
is returned only if it has changed—that is, if the previously requested object’s entity tag
does not match the entity tag of the current version on the server. Chapter 7 discusses
conditional requests in detail.
Type
Request header
Basic Syntax
If-Match: ("*" | 1# entity-tag)
Example
If-Match: "11e92a-457b-31345aa"
If-None-Match
The If-None-Match header, like all the If headers, can be used to make a request condi-
tional. The client supplies the server with a list of entity tags, and the server compares those
tags against the entity tags it has for the resource, returning the resource only if none
match.
This allows a cache to update resources only if they have changed. Using the If-None-
Match header, a cache can use a single request to both invalidate the entities it has and
receive the new entity in the response. Chapter 7 discusses conditional requests in detail.
Type
Request header
Basic Syntax
If-None-Match: ("*" | 1# entity-tag)
Example
If-None-Match: "11e92a-457b-31345aa"
If-Range
The If-Range header, like all the If headers, can be used to make a request conditional. It is
used when an application has a copy of a range of a resource, to revalidate the range or get
the complete resource if the range is no longer valid. Chapter 7 discusses conditional
requests in detail.
Type
Request header
Basic Syntax
If-Range: (HTTP-date | entity-tag)
520 |Appendix C: HTTP Header Reference
Examples
If-Range: Tue, 3 Oct 1997 02:15:31 GMT
If-Range: "11e92a-457b-3134b5aa"
If-Unmodified-Since
The If-Unmodified-Since header is the twin of the If-Modified-Since header. Including it in
a request makes the request conditional. The server should look at the date value of the
header and return the object only if it has not been modified since the date provided.
Chapter 7 discusses conditional requests in detail.
Type
Request header
Basic Syntax
If-Unmodified-Since: HTTP-date
Example
If-Unmodified-Since: Thu, 03 Oct 1997 17:15:00 GMT
Last-Modified
The Last-Modified header tries to provide information about the last time this entity was
changed. This could mean a lot of things. For example, resources typically are files on a
server, so the Last-Modified value could be the last-modified time provided by the server’s
filesystem. On the other hand, for dynamically created resources such as those created by
scripts, the Last-Modified value could be the time the response was created.
Servers need to be careful that the Last-Modified time is not in the future. HTTP/1.1
servers should reset the Last-Modified time if it is later than the value that would be sent in
the Date header.
Type
Entity header
Basic Syntax
Last-Modified: HTTP-date
Example
Last-Modified: Thu, 03 Oct 1997 17:15:00 GMT
Location
The Location header is used by servers to direct clients to the location of a resource that
either was moved since the client last requested it or was created in response to the request.
Type
Response header
Basic Syntax
Location: absoluteURL
Example
Location: http://www.hotbot.com
Pragma |521
Max-Forwards
This header is used only with the TRACE method, to limit the number of proxies or other
intermediaries that a request goes through. Its value is an integer. Each application that
receives a TRACE request with this header should decrement the value before it forwards
the request along.
If the value is zero when the application receives the request, it should send back a 200 OK
response to the request, with an entity body containing the original request. If the Max-
Forwards header is missing from a TRACE request, assume that there is no maximum
number of forwards.
For other HTTP methods, this header should be ignored. See “Methods” in Chapter 3 for
more on the TRACE method.
Type
Request header
Basic Syntax
Max-Forwards: 1*DIGIT
Example
Max-Forwards: 5
MIME-Version
MIME is HTTP’s cousin. While they are radically different, some HTTP servers do
construct messages that are valid under the MIME specification. When this is the case, the
MIME-Version header can be supplied by the server.
This header has never been part of the official specification, although it is mentioned in the
HTTP/1.0 specification. Many older servers send messages with this header, however,
those messages often are not valid MIME messages, making this header both confusing and
impossible to trust.
Type
Extension general header
Basic Syntax
MIME-Version: DIGIT "." DIGIT
Example
MIME-Version: 1.0
Pragma
The Pragma header is used to pass directions along with the message. These directions
could be almost anything, but often they are used to control caching behavior. Proxies and
gateways must not remove the Pragma header, because it could be intended for all applica-
tions that receive the message.
The most common form of Pragma, Pragma: no-cache, is a request header that forces
caches to request or revalidate the document from the origin server even when a fresh copy
is available in the cache. It is sent by browsers when users click on the Reload/Refresh
button. Many servers send Pragma: no-cache as a response header (as an equivalent to
522 |Appendix C: HTTP Header Reference
Cache-Control: no-cache), but despite its common use, this behavior is technically
undefinded. Not all applications support Pragma response headers.
Chapter 7 discusses the Pragma header and how it is used by HTTP/1.0 applications to
control caches.
Type
Request header
Basic Syntax
Pragma: 1# pragma-directive*
Example
Pragma: no-cache
Proxy-Authenticate
The Proxy-Authenticate header functions like the WWW-Authenticate header. It is used by
proxies to challenge an application sending a request to authenticate itself. The full details
of this challenge/response, and other security mechanisms of HTTP, are discussed in detail
in Chapter 14.
If an HTTP/1.1 proxy server is sending a 407 Proxy Authentication Required response, it
must include the Proxy-Authenticate header.
Proxies and gateways must be careful in interpreting all the Proxy headers. They generally
are hop-by-hop headers, applying only to the current connection. For instance, the Proxy-
Authenticate header requests authentication for the current connection.
Type
Response header
Basic Syntax
Proxy-Authenticate: challenge
Example
Proxy-Authenticate: Basic realm="Super Secret Corporate Financial
Documents"
Proxy-Authorization
The Proxy-Authorization header functions like the Authorization header. It is used by
client applications to respond to Proxy-Authenticate challenges. See Chapter 14 for more
on how the challenge/response security mechanism works.
Type
Request header
Basic Syntax
Proxy-Authorization: credentials
Example
Proxy-Authorization: Basic YnJpYW4tdG90dHk6T3ch
* The only specification-defined Pragma directive is “no-cache”; however, you may run into other Pragma
headers that have been defined as extensions to the specification.
Public |523
Proxy-Connection
The Proxy-Connection header was meant to have similar semantics to the HTTP/1.0
Connection header. It was to be used between clients and proxies to specify options about
the connections (chiefly keep-alive connections).*It is not a standard header and is viewed
as an ad hoc header by the standards committee. However, it is widely used by browsers
and proxies.
Browser implementors created the Proxy-Connection header to solve the problem of a
client sending an HTTP/1.0 Connection header that gets blindly forwarded by a dumb
proxy. A server receiving the blindly forwarded Connection header could confuse the capa-
bilities of the client connection with those of the proxy connection.
The Proxy-Connection header is sent instead of the Connection header when the client
knows that it is going through a proxy. Because servers don’t recognize the Proxy-
Connection header, they ignore it, allowing dumb proxies that blindly forward the header
to do so without causing harm.
The problem with this solution occurs if there is more than one proxy in the path of the
client to the server. If the first one blindly forwards the header to the second, which under-
stands it, the second proxy can suffer from the same confusion the server did with the
Connection header.
This is the problem that the HTTP working group had with this solution—they saw it as a
hack that solved the case of a single proxy, but not the bigger problem. Nonetheless, it
does handle some of the more common cases, and because older versions of both Netscape
Navigator and Microsoft Internet Explorer implement it, proxy implementors need to deal
with it. See Chapter 4 for more information.
Type
General header
Basic Syntax
Proxy-Connection: 1# (connection-token)
Example
Proxy-Connection: close
Public
The Public header allows a server to tell a client what methods it supports. These methods
can be used in future requests by the client. Proxies need to be careful when they receive a
response from a server with the Public header. The header indicates the capabilities of the
server, not the proxy, so the proxy needs to either edit the list of methods in the header or
remove the header before it sends the response to the client.
Type
Response header
* See Chapter 4 for more on keep-alive and persistent connections.
524 |Appendix C: HTTP Header Reference
Notes
This header is not defined in RFC 2616. It was previously defined in RFC
2068, an earlier draft of the HTTP/1.1 specification, but it has since been
removed from the official specification.
Basic Syntax
Public: 1# HTTP-method
Example
Public: OPTIONS, GET, HEAD, TRACE, POST
Range
The Range header is used in requests for parts or ranges of an entity. Its value indicates the
range of the entity that is included in the message.
Requests for ranges of a document allow for more efficient requests of large objects (by
requesting them in segments) or for recovery from failed transfers (allowing a client to
request the range of the resource that did not make it). Range requests and the headers that
make the requests possible are discussed in detail in Chapter 15.
Type
Entity header
Example
Range: bytes=500-1500
Referer
The Referer header is inserted into client requests to let the server know where the client
got the URL from. This is a voluntary effort, for the server’s benefit; it allows the server to
better log the requests or perform other tasks. The misspelling of “Referer” hearkens back
to the early days of HTTP, to the frustration of English-speaking copyeditors throughout
the world.
What your browser does is fairly simple. If you get home page A and click on a link to go to
home page B, your browser will insert a Referer header in the request with value A. Referer
headers are inserted by your browser only when you click on links; requests for URLs you
type in yourself will not contain a Referer header.
Because some pages are private, there are some privacy concerns with this header. While
some of this is unwarranted paranoia, this header does allow web servers and their admin-
istrators to see where you came from, potentially allowing them to better track your
surfing. As a result, the HTTP/1.1 specification recommends that application writers allow
the user to decide whether this header is transmitted.
Type
Request header
Basic Syntax
Referer: (absoluteURL | relativeURL)
Example
Referer: http://www.inktomi.com/index.html
Set-Cookie |525
Retry-After
Servers can use the Retry-After header to tell a client when to retry its request for a
resource. It is used with the 503 Service Unavailable status code to give the client a specific
date and time (or number of seconds) at which it should retry its request.
A server can also use this header when it is redirecting clients to resources, giving the client
a time to wait before making a request on the resource to which it is redirected.*This can
be very useful to servers that are creating dynamic resources, allowing the server to redirect
the client to the newly created resource but giving time for the resource to be created.
Type
Response header
Basic Syntax
Retry-After: (HTTP-date | delta-seconds)
Examples
Retry-After: Tue, 3 Oct 1997 02:15:31 GMT
Retry-After: 120
Server
The Server header is akin to the User-Agent header; it provides a way for servers to identify
themselves to clients. Its value is the server name and an optional comment about the
server.
Because the Server header identifies the server product and can contain additional
comments about the product, its format is somewhat free-form. If you are writing software
that depends on how a server identifies itself, you should experiment with the server soft-
ware to see what it sends back, because these tokens vary from product to product and
release to release.
As with the User-Agent header, don’t be surprised if an older proxy or gateway inserts what
amounts to a Via header in the Server header itself.
Type
Response header
Basic Syntax
Server: 1* (product | comment)
Examples
Server: Microsoft-Internet-Information-Server/1.0
Server: websitepro/1.1f (s/n wpo-07d0)
Server: apache/1.2b6 via proxy gateway CERN-HTTPD/3.0 libwww/2.13
Set-Cookie
The Set-Cookie header is the partner to the Cookie header; in Chapter 11, we discuss the
use of this header in detail.
* See “Redirection status codes and reason phrases” in Chapter 3 for more on server redirect responses.
526 |Appendix C: HTTP Header Reference
Type
Extension response header
Basic Syntax
Set-Cookie: command
Examples
Set-Cookie: lastorder=00183; path=/orders
Set-Cookie: private_id=519; secure
Set-Cookie2
The Set-Cookie2 header is an extension of the Set-Cookie header; in Chapter 11, we
discuss the use of this header in detail.
Type
Extension response header
Basic Syntax
Set-Cookie2: command
Examples
Set-Cookie2: ID="29046"; Domain=".joes-hardware.com"
Set-Cookie2: color=blue
TE
The poorly named TE header functions like the Accept-Encoding header, but for transfer
encodings (it could have been named Accept-Transfer-Encoding, but it wasn’t). The TE
header also can be used to indicate whether a client can handle headers in the trailer of a
response that has been through the chunked encoding. See Chapter 15 for more on the TE
header, chunked encoding, and trailers.
Type
Request header
Notes
If the value is empty, only the chunked transfer encoding is acceptable.
The special token “trailers” indicates that trailer headers are acceptable
in a chunked response.
Basic Syntax
TE: # (transfer-codings)
transfer-codings= "trailers" | (transfer-extension [accept-params])
Examples
TE:
TE: chunked
Trailer
The Trailer header is used to indicate which headers are present in the trailer of a message.
Chapter 15 discusses chunked encodings and trailers in detail.
Type
General header
UA-(CPU, Disp, OS, Color, Pixels) |527
Basic Syntax
Trailer: 1#field-name
Example
Trailer: Content-Length
Title
The Title header is a non-specification header that is supposed to give the title of the entity.
This header was part of an early HTTP/1.0 extension and was used primarily for HTML
pages, which have clear title markers that servers can use. Because many, if not most,
media types on the Web do not have such an easy way to extract a title, this header has
limited usefulness. As a result, it never made it into the official specification, though some
older servers on the Net still send it faithfully.
Type
Response header
Notes
The Title header is not defined in RFC 2616. It was originally defined in
the HTTP/1.0 draft definition (http://www.w3.org/Protocols/HTTP/
HTTP2.html) but has since been removed from the official specification.
Basic Syntax
Title: document-title
Example
Title: CNN Interactive
Transfer-Encoding
If some encoding had to be performed to transfer the HTTP message body safely, the
message will contain the Transfer-Encoding header. Its value is a list of the encodings that
were performed on the message body. If multiple encodings were performed, they are listed
in order.
The Transfer-Encoding header differs from the Content-Encoding header because the
transfer encoding is an encoding that was performed by a server or other intermediary
application to transfer the message.
Transfer encodings are discussed in Chapter 15.
Type
General header
Basic Syntax
Transfer-Encoding: 1# transfer-coding
Example
Transfer-Encoding: chunked
UA-(CPU, Disp, OS, Color, Pixels)
These User-Agent headers are nonstandard and no longer common. They provide informa-
tion about the client machine that could allow for better content selection by a server. For
528 |Appendix C: HTTP Header Reference
instance, if a server knew that a user’s machine had only an 8-bit color display, the server
could select images that were optimized for that type of display.
With any header that gives information about the client that otherwise would be unavail-
able, there are some security concerns (see Chapter 14 for more information).
Type
Extension request headers
Notes
These headers are not defined in RFC 2616, and their use is frowned
upon.
Basic Syntax
"UA" "-" ("CPU" | "Disp" | "OS" | "Color" | "Pixels") ":" machine-value
machine-value = (cpu | screensize | os-name | display-color-depth)
Examples
UA-CPU: x86 CPU of client’s machine
UA-Disp: 640, 480, 8 Size and color depth of client’s display
UA-OS: Windows 95 Operating system of client machine
UA-Color: color8 Color depth of client’s display
UA-Pixels: 640x480 Size of client’s display
Upgrade
The Upgrade header provides the sender of a message with a means of broadcasting the
desire to use another, perhaps completely different, protocol. For instance, an HTTP/1.1
client could send an HTTP/1.0 request to a server and include an Upgrade header with the
value “HTTP/1.1”, allowing the client to test the waters and see whether the server speaks
HTTP/1.1.
If the server is capable, it can send an appropriate response letting the client know that it is
okay to use the new protocol. This provides an efficient way to move to other protocols.
Most servers currently are only HTTP/1.0-compliant, and this strategy allows a client to
avoid confusing a server with too many HTTP/1.1 headers until it determines whether the
server is indeed capable of speaking HTTP/1.1.
When a server sends a 101 Switching Protocols response, it must include this header.
Type
General header
Basic Syntax
Upgrade: 1# protocol
Example
Upgrade: HTTP/2.0
User-Agent
The User-Agent header is used by client applications to identify themselves, much like the
Server header for servers. Its value is the product name and possibly a comment describing
the client application.
Via |529
This header’s format is somewhat free-form. Its value varies from client product to product
and release to release. This header sometimes even contains information about the machine
on which the client is running.
As with the Server header, don’t be surprised if older proxy or gateway applications insert
what amounts to a Via header in the User-Agent header itself.
Type;
Request header
Basic Syntax
User-Agent: 1* (product | comment)
Example
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)
Vary
The Vary header is used by servers to inform clients what headers from a client’s request
will be used in server-side negotiation.*Its value is a list of headers that the server looks at
to determine what to send the client as a response.
An example of this would be a server that sends special HTML pages based on your web
browser’s features. A server sending these special pages for a URL would include a Vary
header that indicated that it looked at the User-Agent header of the request to determine
what to send as a response.
The Vary header also is used by caching proxies; see Chapter 7 for more on how the Vary
header relates to cached HTTP responses.
Type
Response header
Basic Syntax
Vary: ("*" | 1# field-name)
Example
Vary: User-Agent
Via
The Via header is used to trace messages as they pass through proxies and gateways. It is
an informational header that can be used to see what applications are handling requests
and responses.
When a message passes through an HTTP application on its way to a client or a server, that
application can use the Via header to tag the message as having gone via it. This is an
HTTP/1.1 header; many older applications insert a Via-like string in the User-Agent or
Server headers of requests and responses.
If the message passes through multiple in-between applications, each one should tack on
its Via string. The Via header must be inserted by HTTP/1.1 proxies and gateways.
* See Chapter 17 for more on content negotiation.
530 |Appendix C: HTTP Header Reference
Type
General header
Basic Syntax
*
Via: 1# (received-protocol received-by [comment])
Example
Via: 1.1 joes-hardware.com (Joes-Server/1.0)
The above says that the message passed through the Joes Server Version
1.0 software running on the machine joes-hardware.com. Joe’s Server
was speaking HTTP 1.1. The Via header should be formatted like this:
HTTP-Version machine-hostname (Application-Name-Version)
Warning
The Warning header is used to give a little more information about what happened during
a request. It provides the server with a way to send additional information that is not in the
status code or reason phrase. Several warning codes are defined in the HTTP/1.1
specification:
101 Response Is Stale
When a response message is known to be stale—for instance, if the origin server is
unavailable for revalidation—this warning must be included.
111 Revalidation Failed
If a cache attempts to revalidate a response with an origin server and the revalidation
fails because the cache cannot reach the origin server, this warning must be included in
the response to the client.
112 Disconnected Operation
An informative warning; should be used if a cache’s connectivity to the network is
removed.
113 Heuristic Expiration
Caches must include this warning if their freshness heuristic is greater than 24 hours
and they are returning a response with an age greater than 24 hours.
199 Miscellaneous Warning
Systems receiving this warning must not take any automated response; the message
may and probably should contain a body with additional information for the user.
214 Transformation Applied
Must be added by any intermediate application, such as a proxy, if the application
performs any transformation that changes the content encoding of the response.
299 Miscellaneous Persistent Warning
Systems receiving this warning must not take any automated reaction; the error may
contain a body with more information for the user.
* See the HTTP/1.1 specification for the complete Via header syntax.
X-Pad |531
Type
Response header
Basic Syntax
Warning: 1# warning-value
Example
Warning: 113
WWW-Authenticate
The WWW-Authenticate header is used in 401 Unauthorized responses to issue a chal-
lenge authentication scheme to the client. Chapter 14 discusses the WWW-Authenticate
header and its use in HTTP’s basic challenge/response authentication system.
Type
Response header
Basic Syntax
WWW-Authenticate: 1# challenge
Example
WWW-Authenticate: Basic realm="Your Private Travel Profile"
X-Cache
The X headers are all extension headers. The X-Cache header is used by Squid to inform a
client whether a resource is available.
Type
Extension response header
Example
X-Cache: HIT
X-Forwarded-For
This header is used by many proxy servers (e.g., Squid) to note whom a request has been
forwarded for. Like the Client-ip header mentioned earlier, this request header notes the
address from which the request originates.
Type
Extension request header
Basic Syntax
X-Forwarded-For: addr
Example
X-Forwarded-For: 64.95.76.161
X-Pad
This header is used to overcome a bug related to response header length in some browsers;
it pads the response message headers with extra bytes to work around the bug.
532 |Appendix C: HTTP Header Reference
Type
Extension general header
Basic Syntax
X-Pad: pad-text
Example
X-Pad: bogosity
X-Serial-Number
The X-Serial-Number header is an extension header. It was used by some older HTTP
applications to insert the serial number of the licensed software in the HTTP message.
Its use has pretty much died out, but it is listed here as an example of the X headers that are
out there.
Type
Extension general header
Basic Syntax
X-Serial-Number: serialno
Example
X-Serial-Number: 010014056
533
APPENDIX D
MIME Types
MIME media types (MIME types, for short) are standardized names that describe the
contents of a message entity body (e.g., text/html, image/jpeg). This appendix
explains how MIME types work, how to register new ones, and where to go for more
information.
In addition, this appendix contains 10 convenient tables, detailing hundreds of
MIME types, gathered from many sources around the globe. This may be the most
detailed tabular listing of MIME types ever compiled. We hope these tables are use-
ful to you.
In this appendix, we will:
• Outline the primary reference material, in “Background.”
• Explain the structure of MIME types, in “MIME Type Structure.”
• Show you how to register MIME types, in “MIME Type IANA Registration.”
• Make it easier for you to look up MIME types.
The following MIME type tables are included in this appendix:
• application/*—Table D-3
• audio/*—Table D-4
• chemical/*—Table D-5
• image/*—Table D-6
• message/*—Table D-7
• model/*—Table D-8
• multipart/*—Table D-9
• text/*—Table D-10
• video/*—Table D-11
• Other—Table D-12
534 |Appendix D: MIME Types
Background
MIME types originally were developed for multimedia email (MIME stands for Mul-
tipurpose Internet Mail Extensions), but they have been reused for HTTP and sev-
eral other protocols that need to describe the format and purpose of data objects.
MIME is defined by five primary documents:
RFC 2045, “MIME: Format of Internet Message Bodies”
Describes the overall MIME message structure, and introduces the Content-
Type header, borrowed by HTTP
RFC 2046, “MIME: Media Types”
Introduces MIME types and their structure
RFC 2047, “MIME: Message Header Extensions for Non-ASCII Text”
Defines ways to include non-ASCII characters in headers
RFC 2048, “MIME: Registration Procedures”
Defines how to register MIME values with the Internet Assigned Numbers
Authority (IANA)
RFC 2049, “MIME: Conformance Criteria and Examples”
Details rules for compliance, and provides examples
For the purposes of HTTP, we are most interested in RFC 2046 (Media Types) and
RFC 2048 (Registration Procedures).
MIME Type Structure
Each MIME media type consists of a type, a subtype, and a list of optional parame-
ters. The type and subtype are separated by a slash, and the optional parameters
begin with a semicolon, if they are present. In HTTP, MIME media types are widely
used in Content-Type and Accept headers. Here are a few examples:
Content-Type: video/quicktime
Content-Type: text/html; charset="iso-8859-6"
Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p
Accept: image/gif
Discrete Types
MIME types can directly describe the object type, or they can describe collections or
packages of other object types. If a MIME type describes an object type directly, it is
adiscrete type. These include text files, videos, and application-specific file formats.
Composite Types
If a MIME type describes a collection or encapsulation of other content, the MIME
type is called a composite type. A composite type describes the format of the enclosing
MIME Type Structure |535
package. When the enclosing package is opened, each enclosed object will have its
own type.
Multipart Types
Multipart media types are composite types. A multipart object consists of multiple
component types. Here’s an example of multipart/mixed content, where each com-
ponent has its own MIME type:
Content-Type: multipart/mixed; boundary=unique-boundary-1
--unique-boundary-1
Content-type: text/plain; charset=US-ASCII
Hi there, I'm some boring ASCII text...
--unique-boundary-1
Content-Type: multipart/parallel; boundary=unique-boundary-2
--unique-boundary-2
Content-Type: audio/basic
... 8000 Hz single-channel mu-law-format
audio data goes here ...
--unique-boundary-2
Content-Type: image/jpeg
... image data goes here ...
--unique-boundary-2--
--unique-boundary-1
Content-type: text/enriched
This is <bold><italic>enriched.</italic></bold>
<smaller>as defined in RFC 1896</smaller>
Isn't it <bigger><bigger>cool?</bigger></bigger>
--unique-boundary-1
Content-Type: message/rfc822
From: (mailbox in US-ASCII)
To: (address in US-ASCII)
Subject: (subject in US-ASCII)
Content-Type: Text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: Quoted-printable
... Additional text in ISO-8859-1 goes here ...
--unique-boundary-1--
536 |Appendix D: MIME Types
Syntax
As we stated earlier, MIME types consist of a primary type, a subtype, and an
optional list of parameters.
The primary type can be a predefined type, an IETF-defined extension token, or an
experimental token (beginning with “x-”). Some common primary types are
described in Table D-1.
Subtypes can be primary types (as in “text/text”), IANA-registered subtypes, or
experimental extension tokens (beginning with “x-”).
Types and subtypes are made up of a subset of US-ASCII characters. Spaces and cer-
tain reserved grouping and punctuation characters, called “tspecials,” are control
characters and are forbidden from type and subtype names.
The grammar from RFC 2046 is shown below:
TYPE := "application" | "audio" | "image" | "message" | "multipart" |
"text" | "video" | IETF-TOKEN | X-TOKEN
SUBTYPE := IANA-SUBTOKEN | IETF-TOKEN | X-TOKEN
IETF-TOKEN := <extension token with RFC and registered with IANA>
IANA-SUBTOKEN := <extension token registered with IANA>
X-TOKEN := <"X-" or "x-" prefix, followed by any token>
PARAMETER := TOKEN "=" VALUE
VALUE := TOKEN / QUOTED-STRING
TOKEN := 1*<any (US-ASCII) CHAR except SPACE, CTLs, or TSPECIALS>
TSPECIALS := "(" | ")" | "<" | ">" | "@" |
"," | ";" | ":" | "\" | <"> |
"/" | "[" | "]" | "?" | "="
Table D-1. Common primary MIME types
Type Description
application Application-specific content format (discrete type)
audio Audio format (discrete type)
chemical Chemical data set (discrete IETF extension type)
image Image format (discrete type)
message Message format (composite type)
model 3-D model format (discrete IETF extension type)
multipart Collection of multiple objects (composite type)
text Text format (discrete type)
video Video movie format (discrete type)
MIME Type IANA Registration |537
MIME Type IANA Registration
The MIME media type registration process is described in RFC 2048. The goal of the
registration process is to make it easy to register new media types but also to provide
some sanity checking to make sure the new types are well thought out.
Registration Trees
MIME type tokens are split into four classes, called “registration trees,” each with its
own registration rules. The four trees—IETF, vendor, personal, and experimental—
are described in Table D-2.
Registration Process
Read RFC 2048 carefully for the details of MIME media type registration.
The basic registration process is not a formal standards process; it’s just an adminis-
trative procedure intended to sanity check new types with the community, and record
them in a registry, without much delay. The process follows the following steps:
1. Present the media type to the community for review.
Send a proposed media type registration to the ietf-types@iana.org mailing list
for a two-week review period. The public posting solicits feedback about the
choice of name, interoperability, and security implications. The “x-” prefix spec-
ified in RFC 2045 can be used until registration is complete.
Table D-2. Four MIME media type registration trees
Registration tree Example Description
IETF text/html
(HTML text)
The IETF tree is intended for types that are of general significance to the
Internet community. New IETF tree media types require approval by the
Internet Engineering Steering Group (IESG) and an accompanying
standards-track RFC.
IETF tree types have no periods (.) in tokens.
Vendor
(vnd.)
image/vnd.fpx
(Kodak FlashPix image)
The vendor tree is intended for media types used by commercially available
products. Public review of new vendor types is encouraged but not
required.
Vendor tree types begin with “vnd.”.
Personal/Vanity
(prs.)
image/prs.btif
(internal check-
management format
used by Nations Bank)
Private, personal, or vanity media types can be registered in the personal
tree. These media types will not be distributed commercially.
Personal tree types begin with “prs.”.
Experimental
(x- or x.)
application/x-tar
(Unix tar archive)
The experimental tree is for unregistered or experimental media types.
Because it’s relatively simple to register a new vendor or personal media
type, software should not be distributed widely using x- types.
Experimental tree types begin with “x.” or “x-”.
538 |Appendix D: MIME Types
2. IESG approval (for IETF tree only).
If the media type is being registered in the IETF tree, it must be submitted to the
IESG for approval and must have an accompanying standards-track RFC.
3. IANA registration.
As soon as the media type meets the approval requirements, the author can sub-
mit the registration request to the IANA, using the email template in
Example D-1 and mailing the information to ietf-types@iana.org. The IANA will
register the media type and make the media type application available to the
community at http://www.isi.edu/in-notes/iana/assignments/media-types/.
Registration Rules
The IANA will register media types in the IETF tree only in response to a communi-
cation from the IESG stating that a given registration has been approved.
Vendor and personal types will be registered by the IANA automatically and with-
out any formal review as long as the following minimal conditions are met:
1. Media types must function as actual media formats. Types that act like transfer
encodings or character sets may not be registered as media types.
2. All media types must have proper type and subtype names. All type names must
be defined by standards-track RFCs. All subtype names must be unique, must
conform to the MIME grammar for such names, and must contain the proper
tree prefixes.
3. Personal tree types must provide a format specification or a pointer to one.
4. Any security considerations given must not be obviously bogus. Everyone who is
developing Internet software needs to do his part to prevent security holes.
Registration Template
The actual IANA registration is done via email. You complete a registration form
using the template shown in Example D-1, and mail it to ietf-types@iana.org.*
* The lightly structured nature of the form makes the submitted information fine for human consumption but
difficult for machine processing. This is one reason why it is difficult to find a readable, well-organized sum-
mary of MIME types, and the reason we created the tables that end this appendix.
Example D-1. IANA MIME registration email template
To: ietf-types@iana.org
Subject: Registration of MIME media type XXX/YYY
MIME media type name:
MIME Type Tables |539
MIME Media Type Registry
The submitted forms are accessible from the IANA web site (http://www.iana.org). At
the time of writing, the actual database of MIME media types is stored on an ISI web
server, at http://www.isi.edu/in-notes/iana/assignments/media-types/.
The media types are stored in a directory tree, structured by primary type and sub-
type, with one leaf file per media type. Each file contains the email submission.
Unfortunately, each person completes the registration template slightly differently,
so the quality and format of information varies across submissions. (In the tables in
this appendix, we tried to fill in the holes omitted by registrants.)
MIME Type Tables
This section summarizes hundreds of MIME types in 10 tables. Each table lists the
MIME media types within a particular primary type (image, text, etc.).
MIME subtype name:
Required parameters:
Optional parameters:
Encoding considerations:
Security considerations:
Interoperability considerations:
Published specification:
Applications which use this media type:
Additional information:
Magic number(s):
File extension(s):
Macintosh File Type Code(s):
Person & email address to contact for further information:
Intended usage:
(One of COMMON, LIMITED USE or OBSOLETE)
Author/Change controller:
(Any other information that the author deems interesting may be added below this line.)
Example D-1. IANA MIME registration email template (continued)
540 |Appendix D: MIME Types
The information is gathered from many sources, including the IANA media type reg-
istry, the Apache mime.types file, and assorted Internet web pages. We spent several
days refining the data, plugging holes, and including descriptive summaries from
cross-references to make the data more useful.
This may well be the most detailed tabular listing of MIME types ever compiled. We
hope you find it handy!
application/*
Table D-3 describes many of the application-specific MIME media types.
Table D-3. “Application” MIME types
MIME type Description Extension Contact and reference
application/activemessage Supports the Active Mail groupware
system.
“Active Mail: A Framework for Inte-
grated Groupware Applications” in
Readings in Groupware and
Computer-Supported Cooperative
Work, Ronald M. Baecker, ed.,
Morgan Kaufmann, ISBN
1558602410
application/andrew-inset Supports the creation of multimedia
content with the Andrew toolkit.
ez Multimedia Applications Develop-
ment with the Andrew Toolkit,
Nathaniel S. Borenstein, Prentice
Hall, ASIN 0130366331
nsb@bellcore.com
application/applefile Permits MIME-based transmission of
data with Apple/Macintosh-specific
information, while allowing general
access to nonspecific user data.
RFC 1740
application/atomicmail ATOMICMAIL was an experimental
research project at Bellcore, designed
for including programs in electronic
mail messages that are executed
when mail is read. ATOMICMAIL is
rapidly becoming obsolete in favor of
safe-tcl.
“ATOMICMAIL Language Reference
Manual,” Nathaniel S. Borenstein,
Bellcore Technical Memorandum
TM ARH-018429
application/batch-SMTP Defines a MIME content type suitable
for tunneling an ESMTP mail transac-
tion through any MIME-capable
transport.
RFC 2442
application/beep+xml Supports the interaction protocol
called BEEP. BEEP permits simulta-
neous and independent exchanges of
MIME messages between peers,
where the messages usually are XML-
structured text.
RFC 3080
MIME Type Tables |541
application/cals-1840 Supports MIME email exchanges of
U.S. Department of Defense digital
datathat was previouslyexchanged by
tapem, as defined by MIL-STD-1840.
RFC 1895
application/commonground Common Ground is an electronic doc-
ument exchange and distribution pro-
gramthat lets users create documents
that anyone can view, search, and
print, without requiring that they
have the creating applications or
fonts on their systems.
Nick Gault
No Hands Software
ngault@nohands.com
application/cybercash Supportscredit card paymentthrough
the CyberCash protocol. When a user
starts payment, a message is sent by
the merchant to the customer as the
body of a message of MIME type
application/cybercash.
RFC 1898
application/dca-rft IBM Document Content Architecture. “IBM Document Content Architec-
ture/Revisable Form Text Refer-
ence,” document number SC23-
0758-1, International Business
Machines
application/dec-dx DEC Document Transfer Format. “Digital Document Transmission
(DX) Technical Notebook,” docu-
ment number EJ29141-86, Digital
Equipment Corporation
application/dvcs Supports the protocols used by a Data
Validation and Certification Server
(DVCS), which acts as a trusted third
party in a public-key security
infrastructure.
RFC 3029
application/EDI-Consent Supports bilateral trading via elec-
tronic data interchange (EDI), using
nonstandard specifications.
http //www.isi.edu/in-notes/iana/
assignments/media-types/
application/EDI-Consent
application/EDI-X12 Supports bilateral trading via elec-
tronic data interchange (EDI), using
the ASC X12 EDI specifications.
http //www.isi.edu/in-notes/iana/
assignments/media-types/
application/EDI-X12
application/EDIFACT Supports bilateral trading via elec-
tronic data interchange (EDI), using
the EDIFACT specifications.
http //www.isi.edu/in-notes/iana/
assignments/media-types/
application/EDIFACT
application/eshop Unknown. Steve Katz
System Architecture Shop
steve_katz@eshop.com
application/font-tdpfr Definesa Portable Font Resource (PFR)
that contains a set of glyph shapes,
each associated with a character code.
RFC 3073
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
542 |Appendix D: MIME Types
application/http Used to enclose a pipeline of one or
more HTTP request or response mes-
sages (not intermixed).
RFC 2616
application/hyperstudio Supports transfer of HyperStudio edu-
cational hypermedia files.
stk http //www.hyperstudio.com
application/iges A commonly used format for CAD
model interchange.
“ANS/US PRO/IPO-100”
U.S. Product Data Association
2722 Merrilee Drive, Suite 200
Fairfax, VA 22031-4499
application/index
application/index.cmd
application/index.obj
application/index.response
application/index.vnd
Support the Common Indexing Proto-
col (CIP). CIP is an evolution of the
Whois++ directory service, used to
passindexing information from server
to server in order to redirect and repli-
cate queries through a distributed
database system.
RFC 2652, and RFCs 2651, 1913,
and 1914
application/iotp Supports Internet Open Trading Pro-
tocol (IOTP) messages over HTTP.
RFC 2935
application/ipp Supports Internet Printing Protocol
(IPP) over HTTP.
RFC 2910
application/mac-binhex40 Encodes a string of 8-bit bytes into a
string of 7-bit bytes, which is safer for
some applications (though not quite
as safe as the 6-bit base-64 encoding).
hqx RFC 1341
application/mac-compactpro From Apache mime.types. cpt
application/macwriteii Claris MacWrite II.
application/marc MARC objects are Machine-Readable
Cataloging records—standards for
the representation and communica-
tion of bibliographic and related
information.
mrc RFC 2220
application/mathematica
application/mathematica-old
Supports Mathematica and Math-
Reader numerical analysis software.
nb, ma,
mb
The Mathematica Book, Stephen
Wolfram, Cambridge University
Press, ISBN 0521643147
application/msword Microsoft Word MIME type. doc
application/news-message-id RFCs 822 (message IDs), 1036
(application to news), and 977
(NNTP)
application/news-
transmission
Allows transmission of news articles
by email or other transport.
RFC 1036
application/ocsp-request Supports the Online Certificate Status
Protocol (OCSP), which provides a
way to check on the validity of a digi-
tal certificate without requiring local
certificate revocation lists.
orq RFC 2560
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |543
application/ocsp-response Same as above. ors RFC 2560
application/octet-stream Unclassified binary data. bin,dms,
lha, lzh,
exe, class
RFC 1341
application/oda Used for information encoded accord-
ing to the Office Document Architec-
ture (ODA) standards, using the Office
Document Interchange Format (ODIF)
representation format. The Content-
Type line also should specify an
attribute/value pair that indicates the
document application profile (DAP),
as in:
Content-Type: application/oda;
profile=Q112
oda RFC 1341
ISO 8613; “Information Processing:
Text and Office System; Office Doc-
ument Architecture (ODA) and
Interchange Format (ODIF),” Part
1-8, 1989
application/parityfec Forward error correction parity encod-
ing for RTP data streams.
RFC 3009
application/pdf Adode PDF files. pdf See Portable Document Format Ref-
erenceManual, AdobeSystems, Inc.,
Addison Wesley, ISBN 0201626284
application/pgp-encrypted PGP encrypted data. RFC 2015
application/pgp-keys PGP public-key blocks. RFC 2015
application/pgp-signature PGP cryptographic signature. RFC 2015
application/pkcs10 Public Key Crypto System #10—the
application/pkcs10 body type must be
used to transfer a PKCS #10 certifica-
tion request.
p10 RFC 2311
application/pkcs7-mime Public Key Crypto System #7—this
type is used to carry PKCS #7 objects
of several types including enveloped-
Data and signedData.
p7m RFC 2311
application/pkcs7-signature Public Key Crypto System #7—this
type always contains a single PKCS #7
object of type signedData.
p7s RFC 2311
application/pkix-cert Transports X.509 certificates. cer RFC 2585
application/pkix-crl Transports X.509 certificate revoca-
tion lists.
crl RFC 2585
application/pkixcmp Message format used by X.509 Public
Key Infrastructure Certificate Man-
agement Protocols.
pki RFC 2510
application/postscript An Adobe PostScript graphics file
(program).
ai, ps,
eps
RFC 2046
application/prs.alvestrand.
titrax-sheet
“TimeTracker” program by Harald T.
Alvestrand.
http //domen.uninett.no/~hta/
titrax/
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
544 |Appendix D: MIME Types
application/prs.cww CU-Writer for Windows. cw, cww Dr. Somchai Prasitjutrakul
somchaip@chulkn.car.chula.ac.th
application/prs.nprend Unknown. rnd, rct John M. Doggett
jdoggett@tiac.net
application/remote-printing Contains meta information used
when remote printing, for the printer
cover sheet.
RFC 1486
Marshall T. Rose
mrose@dbc.mtview.ca.us
application/riscos Acorn RISC OS binaries. RISC OS Programmer’s Reference
Manuals, Acorn Computers, Ltd.,
ISBN1852501103
application/sdp SDP is intended for describing live
multimedia sessions for the purposes
of session announcement, session
invitation, and other forms of multi-
media session initiation.
RFC 2327
Henning Schulzrinne
hgs@cs.columbia.edu
application/set-payment
application/set-payment-
initiation
application/set-registration
application/set-registration-
initiation
Supports the SET secure electronic
transaction payment protocol.
http //www.visa.com
http //www.mastercard.com
application/sgml-open-
catalog
Intended for use with systems that
support the SGML Open TR9401:1995
“Entity Management” specification.
SGML Open
910 Beaver Grade Road, #3008
Coraopolis, PA 15109
info@sgmlopen.org
application/sieve Sieve mail filtering script. RFC 3028
application/slate The BBN/Slate document format is
published as part of the standard doc-
umentation set distributed with the
BBN/Slate product.
BBN/Slate Product Mgr
BBN Systems and Technologies
10 Moulton Street
Cambridge, MA 02138
application/smil The Synchronized Multimedia Inte-
gration Language (SMIL) integrates a
set of independent multimedia
objects into a synchronized multi-
media presentation.
smi, smil http //www.w3.org/AudioVideo/
application/tve-trigger Supports embedded URLs in
enhanced television receivers.
“SMPTE: Declarative Data Essence,
Content Level 1,” produced by the
Society of Motion Picture and
Television Engineers
http //www.smpte.org
application/vemmi Enhanced videotex standard. RFC 2122
application/vnd.3M.Post-it-
Notes
Used by the “Post-it® Notes for Inter-
net Designers” Internet control/
plug-in.
pwn http //www.3M.com/psnotes/
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |545
application/vnd.accpac.
simply.aso
Simply Accounting v7.0 and higher.
Files of this type conform to Open
Financial Exchange v1.02
specifications.
aso http //www.ofx.net
application/vnd.accpac.
simply.imp
Used by Simply Accounting v7.0 and
higher, to import its own data.
imp http //www.ofx.net
application/vnd.acucobol ACUCOBOL-GT Runtime. Dovid Lubin
dovid@acucobol.com
application/vnd.aether.imp Supports airtime-efficient Instant
Message communications between
an Instant Messaging service, such as
AOL Instant Messenger, Yahoo! Mes-
senger, or MSN Messenger, and a spe-
cial set of Instant Messaging client
software on a wireless device.
Wireless Instant Messaging Proto-
col (IMP) specification available
from Aether Systems by license
application/vnd.anser-web-
certificate-issue-initiation
Trigger for web browsers to launch
the ANSER-WEB Terminal Client.
cii Hiroyoshi Mori
mori@mm.rd.nttdata.co.jp
application/vnd.anser-web-
funds-transfer-initiation
Same as above. fti Same as above
application/vnd.audiograph AudioGraph. aep Horia Cristian
H.C.Slusanschi@massey.ac.nz
application/vnd.bmi BMI graphics format by CADAM
Systems.
bmi Tadashi Gotoh
tgotoh@cadamsystems.co.jp
application/vnd.
businessobjects
BusinessObjects 4.0 and higher. rep
application/vnd.canon-cpdl
application/vnd.canon-lips
Supports Canon, Inc. office imaging
products.
Shin Muto
shinmuto@pure.cpdc.canon.co.jp
application/vnd.claymore Claymore.exe. cla Ray Simpson
ray@cnation.com
application/vnd.commerce-
battelle
Supports a generic mechanism for
delimiting smart card–based infor-
mation, for digital commerce, identi-
fication, authentication, and
exchange of smart card–based card
holder information.
ica, icf,
icd, icc,
ic0, ic1,
ic2, ic3,
ic4, ic5,
ic6, ic7,
ic8
David C. Applebaum
applebau@131.167.52.15
application/vnd.
commonspace
Allows for proper transmission of
CommonSpace™ documents via
MIME-based processes. Common-
Space is published by Sixth Floor
Media, part of the Houghton-Mifflin
Company.
csp, cst Ravinder Chandhok
chandhok@within.com
application/vnd.contact.cmsg Used for CONTACT software’s CIM
DATABASE.
cdbcmsg Frank Patz
fp@contact.de
http //www.contact.de
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
546 |Appendix D: MIME Types
application/vnd.cosmocaller Allows for files containing connection
parameters to be downloaded from
web sites, invokes the CosmoCaller
application to interpret the parame-
ters, and initiates connections with
the CosmoCallACD server.
cmc Steve Dellutri
sdellutri@cosmocom.com
application/vnd.ctc-posml Continuum Technology’s PosML. pml Bayard Kohlhepp
bayardk@ctcexchange.com
application/vnd.cups-
postscript
application/vnd.cups-raster
application/vnd.cups-raw
Supports Common UNIX Printing Sys-
tem (CUPS) servers and clients.
http //www.cups.org
application/vnd.cybank Proprietary data type for Cybank data. Nor Helmee B. Abd. Halim
helmee@cybank.net
http //www.cybank.net
application/vnd.dna DNA is intended to easily Web-enable
any 32-bit Windows application.
dna Meredith Searcy
msearcy@newmoon.com
application/vnd.dpgraph Used by DPGraph 2000 and Math-
Ware Cyclone.
dpg,
mwc,
dpgraph
David Parker
http //www.davidparker.com
application/vnd.dxr Digital Xpress Reports by PSI
Technologies.
dxr Michael Duffy
miked@psiaustin.com
application/vnd.ecdis-update Supports ECDIS applications. http //www.sevencs.com
application/vnd.ecowin.chart
application/vnd.ecowin.
filerequest
application/vnd.ecowin.
fileupdate
application/vnd.ecowin.series
application/vnd.ecowin.
seriesrequest
application/vnd.ecowin.
seriesupdate
EcoWin. mag Thomas Olsson
thomas@vinga se
application/vnd.enliven Supports delivery of Enliven interac-
tive multimedia.
nml Paul Santinelli
psantinelli@narrative.com
application/vnd.epson.esf Proprietary content for Seiko Epson
QUASS Stream Player.
esf Shoji Hoshina
Hoshina.Shoji@exc.epson.co.jp
application/vnd.epson.msf Proprietary content for Seiko Epson
QUASS Stream Player.
msf Same as above
application/vnd.epson.
quickanime
Proprietary content for Seiko Epson
QuickAnime Player.
qam Yu Gu
guyu@rd.oda.epson.co.jp
application/vnd.epson.salt Proprietary content for Seiko Epson
SimpleAnimeLite Player.
slt Yasuhito Nagatomo
naga@rd.oda.epson.co.jp
application/vnd.epson.ssf Proprietary content for Seiko Epson
QUASS Stream Player.
ssf Shoji Hoshina
Hoshina.Shoji@exc.epson.co.jp
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |547
application/vnd.ericsson.
quickcall
Phone Doubler Quick Call. qcall, qca Paul Tidwell
paul.tidwell@ericsson.com
http //www.ericsson.com
application/vnd.eudora.data Eudora Version 4.3 and later. Pete Resnick
presnick@qualcomm.com
application/vnd.fdf Adobe Forms Data Format. “Forms Data Format,” Technical
Note 5173, Adobe Systems
application/vnd.ffsns Used for application communication
with FirstFloor’s Smart Delivery.
Mary Holstege
holstege@firstfloor.com
application/vnd.FloGraphIt NpGraphIt. gph
application/vnd.framemaker Adobe FrameMaker files. fm, mif,
book
http //www.adobe.com
application/vnd.fsc.
weblaunch
Supports Friendly Software Corpora-
tion’s golf simulation software.
fsc Derek Smith
derek@friendlysoftware.com
application/vnd.fujitsu.oasys
application/vnd.fujitsu.oasys2
Supports Fujitsu’s OASYS software. oas Nobukazu Togashi
togashi@ai.cs.fujitsu.co.jp
application/vnd.fujitsu.oasys2 Supports Fujitsu’s OASYS V2 software. oa2 Same as above
application/vnd.fujitsu.oasys3 Support’s Fujitsu’s OASYSV5 software. oa3 Seiji Okudaira
okudaira@candy.paso.fujitsu.co.jp
application/vnd.fujitsu.
oasysgp
Supports Fujitsu’s OASYS GraphPro
software.
fg5 Masahiko Sugimoto
sugimoto@sz.sel.fujitsu.co.jp
application/vnd.fujitsu.
oasysprs
Support’s Fujitsu’s OASYS Presenta-
tion software.
bh2 Masumi Ogita
ogita@oa.tfl.fujitsu.co.jp
application/vnd.fujixerox.ddd Supports Fuji Xerox’s EDMICS 2000
and DocuFile.
ddd Masanori Onda
Masanori.Onda@fujixerox.co.jp
application/vnd.fujixerox.
docuworks
Supports Fuji Xerox’s DocuWorks Desk
and DocuWorks Viewer software.
xdw Yasuo Taguchi
yasuo.taguchi@fujixerox.co.jp
application/vnd.fujixerox.
docuworks.binder
Supports Fuji Xerox’s DocuWorks Desk
and DocuWorks Viewer software.
xbd Same as above.
application/vnd.fut-misnet Unknown. Jaan Pruulmann
jaan@fut.ee
application/vnd.grafeq Lets users of GrafEq exchange GrafEq
documents through the Web and
email.
gqf, gqs http //www.peda.com
application/vnd.groove-
account
Groove is a peer-to-peer communica-
tion system implementing a virtual
space for small group interaction.
gac Todd Joseph
todd_joseph@groove.net
application/vnd.groove-
identity-message
Same as above. gim Same as above
application/vnd.groove-
injector
Same as above. grv Same as above
application/vnd.groove-tool-
message
Same as above. gtm Same as above
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
548 |Appendix D: MIME Types
application/vnd.groove-tool-
template
Same as above. tpl Same as above
application/vnd.groove-vcard Same as above. vcg Same as above
application/vnd.hhe.lesson-
player
Supports the LessonPlayer and Pre-
sentationEditor software.
les Randy Jones
Harcourt E-Learning
randy_jones@archipelago.com
application/vnd.hp-HPGL HPGL files. The HP-GL/2 and HP RTL Reference
Guide, Addison Wesley, ISBN
0201310147
application/vnd.hp-hpid Supports Hewlett-Packard’s Instant
Delivery Software.
hpi, hpid http //www.instant-delivery.com
application/vnd.hp-hps Supports Hewlett-Packard’s Web-
PrintSmart software.
hps http //www.hp.com/go/
webprintsmart_mimetype_specs/
application/vnd.hp-PCL
application/vnd.hp-PCLXL
PCL printer files. pcl “PCL-PJL Technical Reference Man-
ual Documentation Package,” HP
Part No. 5012-0330
application/vnd.httphone HTTPhone asynchronous voice over IP
system.
Franck LeFevre
franck@k1info.com
application/vnd.hzn-3d-
crossword
Used to encode crossword puzzles by
Horizon, A Glimpse of Tomorrow.
x3d James Minnis
james_minnis@glimpse-of-
tomorrow.com
application/vnd.ibm.
afplinedata
Print Services Facility (PSF), AFP Con-
version and Indexing Facility (ACIF).
Roger Buis
buis@us.ibm.com
application/vnd.ibm.MiniPay MiniPay authentication and payment
software.
mpy Amir Herzberg
amirh@vnet.ibm.com
application/vnd.ibm.modcap Mixed Object Document Content. list3820,
listafp,
afp,
pseg3820
Reinhard Hohensee
rhohensee@vnet.ibm.com
“Mixed Object Document Content
Architecture Reference,” IBM publi-
cation SC31-6802
application/vnd.informix-
visionary
Informix Visionary. vis Christopher Gales
christopher.gales@informix.com
application/vnd.intercon.
formnet
SupportsInterconAssociatesFormNet
software.
xpw, xpx Thomas A. Gurak
assoc@intercon.roc servtech.com
application/vnd.intertrust.
digibox
application/vnd.intertrust.
nncp
Supports InterTrust architecture for
secure electronic commerce and digi-
tal rights management.
InterTrust Technologies
460 Oakmead Parkway
Sunnyvale, CA 94086 USA
info@intertrust.com
http //www.intertrust.com
application/vnd.intu.qbo Intended for use only with Quick-
Books 6.0 (Canada).
qbo Greg Scratchley
greg_scratchley@intuit.com
Format of these files discussed in
the Open Financial Exchange specs,
available from http://www.ofx.net
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |549
application/vnd.intu.qfx Intended for use only with Quicken 99
and following versions.
qfx Same as above
application/vnd.is-xpr Express by Infoseek. xpr Satish Natarajan
satish@infoseek.com
application/vnd.japannet-
directory-service
application/vnd.japannet-
jpnstore-wakeup
application/vnd.japannet-
payment-wakeup
application/vnd.japannet-
registration
application/vnd.japannet-
registration-wakeup
application/vnd.japannet-
setstore-wakeup
application/vnd.japannet-
verification
application/vnd.japannet-
verification-wakeup
Supports Mitsubishi Electric’s Japan-
Net security, authentication, and pay-
ment sofwtare.
Jun Yoshitake
yositake@iss.isl.melco.co.jp
application/vnd.koan Supports the automatic playback of
Koan music files over the Internet, by
helper applications such as SSEYO
Koan Netscape Plugin.
skp, skd,
skm, skt
Peter Cole
pcole@sseyod.demon.co.uk
application/vnd.lotus-1-2-3 Lotus 1-2-3 and Lotus approach. 123,
wk1,
wk3,
wk4
Paul Wattenberger
Paul_Wattenberger@lotus.com
application/vnd.lotus-
approach
Lotus Approach. apr, vew Same as above
application/vnd.lotus-
freelance
Lotus Freelance. prz, pre Same as above
application/vnd.lotus-notes Lotus Notes. nsf, ntf,
ndl, ns4,
ns3, ns2,
nsh, nsg
Michael Laramie
laramiem@btv.ibm.com
application/vnd.lotus-
organizer
Lotus Organizer. or3, or2,
org
Paul Wattenberger
Paul_Wattenberger@lotus.com
application/vnd.lotus-
screencam
Lotus ScreenCam. scm Same as above
application/vnd.lotus-
wordpro
Lotus Word Pro. lwp, sam Same as above
application/vnd.mcd Micro CADAM CAD software. mcd Tadashi Gotoh
tgotoh@cadamsystems.co.jp
http //www.cadamsystems.co.jp
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
550 |Appendix D: MIME Types
application/vnd.
mediastation.cdkey
Supports Media Station’s CDKey
remote CDROM communications
protocol.
cdkey Henry Flurry
henryf@mediastation.com
application/vnd.meridian-
slingshot
Slingshot by Meridian Data. Eric Wedel
Meridian Data, Inc.
5615 Scotts Valley Drive
Scotts Valley, CA 95066
ewedel@meridian-data.com
application/vnd.mif FrameMaker interchange format. mif ftp://ftp.frame.com/pub/techsup/
techinfo/dos/mif4.zip
Mike Wexler
Adobe Systems, Inc
333 W. San Carlos St.
San Jose, CA 95110 USA
mwexler@adobe.com
application/vnd.minisoft-
hp3000-save
NetMail 3000 save format. Minisoft, Inc.
support@minisoft.com
ftp://ftp.3k.com/DOC/ms92-save-
format.txt
application/vnd.mitsubishi.
misty-guard.trustweb
Supports Mitsubishi Electric’s
Trustweb software.
Manabu Tanaka
mtana@iss.isl.melco.co.jp
application/vnd.Mobius.DAF Supports Mobius Management Sys-
tems software.
daf Celso Rodriguez
crodrigu@mobius.com
Greg Chrzczon
gchrzczo@mobius.com
application/vnd.Mobius.DIS Same as above. dis Same as above
application/vnd.Mobius.MBK Same as above. mbk Same as above
application/vnd.Mobius.MQY Same as above. mqy Same as above
application/vnd.Mobius.MSL Same as above. msl Same as above
application/vnd.Mobius.PLC Same as above. plc Same as above
application/vnd.Mobius.TXF Same as above. txf Same as above
application/vnd.motorola.
flexsuite
FLEXsuite™ is a collection of wireless
messaging protocols. This type is used
by the network gateways of wireless
messaging service providers as well as
wireless OSs and applications.
Mark Patton
Motorola Personal Networks Group
fmp014@email.mot.com
FLEXsuite™ specification available
from Motorola under appropriate
licensing agreement
application/vnd.motorola.
flexsuite.adsi
FLEXsuite™ is a collection of wireless
messaging protocols. This type pro-
vides a wireless-friendly format for
enabling various data-encryption
solutions.
Same as above
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |551
application/vnd.motorola.
flexsuite.fis
FLEXsuite™ is a collection of wireless
messaging protocols. This type is a
wireless-friendly format for the effi-
cient delivery of structured informa-
tion (e.g., news, stocks, weather) to a
wireless device.
Same as above
application/vnd.motorola.
flexsuite.gotap
FLEXsuite™ is a collection of wireless
messaging protocols. This type pro-
vides a common wireless-friendly for-
mat for the programming of wireless
device attributes via over-the-air
messages.
Same as above
application/vnd.motorola.
flexsuite.kmr
FLEXsuite™ is a collection of wireless
messaging protocols. This type pro-
vides a wireless-friendly format for
encryption key management.
Same as above
application/vnd.motorola.
flexsuite.ttc
FLEXsuite™ is a collection of wireless
messaging protocols. This type sup-
ports a wireless-friendly format for
the efficient delivery of text using
token text compression.
Same as above
application/vnd.motorola.
flexsuite.wem
FLEXsuite™ is a collection of wireless
messaging protocols. This type pro-
vides a wireless-friendly format for
the communication of Internet email
to wireless devices.
Same as above
application/vnd.mozilla.
xul+xml
Supports the Mozilla Internet applica-
tion suite.
xul Dan Rosen2
dr@netscape.com
application/vnd.ms-artgalry Supports Microsoft’s Art Gallery. cil deansl@microsoft.com
application/vnd.ms-asf ASF is a multimedia file format whose
contents are designed to be streamed
across a network to support distrib-
uted multimedia applications. ASF
content may include any combination
of any media type (e.g., audio, video,
images, URLs, HTML content, MIDI,
2-D and 3-D modeling, scripts, and
objects of various types).
asf Eric Fleischman
ericf@microsoft.com
http //www.microsoft.com/
mind/0997/netshow/netshow.asp
application/vnd.ms-excel Microsoft Excel spreadsheet. xls Sukvinder S. Gill
sukvg@microsoft.com
application/vnd.ms-lrm Microsoft proprietary. lrm Eric Ledoux
ericle@microsoft.com
application/vnd.ms-
powerpoint
Microsoft PowerPoint presentation. ppt Sukvinder S. Gill
sukvg@microsoft.com
application/vnd.ms-project Microsoft Project file. mpp Same as above
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
552 |Appendix D: MIME Types
application/vnd.ms-tnef Identifies an attachment that in gen-
eral would be processable only by a
MAPI-aware application. This type is
an encapsulated format of rich MAPI
properties, such as Rich Text and Icon
information, that may otherwise be
degraded by the messaging transport.
Same as above
application/vnd.ms-works Microsoft Works software. Same as above
application/vnd.mseq MSEQis a compact multimedia format
suitable for wireless devices.
mseq Gwenael Le Bodic
Gwenael.le_bodic@alcatel.fr
http //www.3gpp.org
application/vnd.msign Used by applications implementing
the msign protocol, which requests
signatures from mobile devices.
Malte Borcherding
Malte.Borcherding@brokat.com
application/vnd.music-niff NIFF music files. Cindy Grande
72723.1272@compuserve.com
ftp://blackbox.cartah.washington.
edu/pub/NIFF/NIFF6A.TXT
application/vnd.musician MUSICIANscoringlanguage/encoding
conceived and developed by Renai-
Science Corporation.
mus Robert G. Adams
gadams@renaiscience.com
application/vnd.netfpx Intended for dynamic retrieval of
multiresolutionimage information,as
used by Hewlett-Packard Company
Imaging for Internet.
fpx Andy Mutz
andy_mutz@hp.com
application/vnd.noblenet-
directory
Supports the NobleNet Directory soft-
ware, purchased by RogueWave.
nnd http //www.noblenet.com
application/vnd.noblenet-
sealer
Supports the NobleNet Sealer soft-
ware, purchased by RogueWave.
nns http //www.noblenet.com
application/vnd.noblenet-
web
Supports the NobleNet Web software,
purchased by RogueWave.
nnw http //www.noblenet.com
application/vnd.novadigm.
EDM
Supports Novadigm’s RADIA and EDM
products.
edm Phil Burgard
pburgard@novadigm.com
application/vnd.novadigm.
EDX
Same as above. edx Same as above
application/vnd.novadigm.
EXT
Same as above. ext Same as above
application/vnd.osa.
netdeploy
Supports the Open Software Associ-
ates netDeploy application deploy-
ment software.
ndc Steve Klos
stevek@osa.com
http //www.osa.com
application/vnd.palm Used by PalmOS system software and
applications—this new type, “appli-
cation/vnd.palm,” replaces the old
type “application/x-pilot.”
prc, pdb,
pqa, oprc
Gavin Peacock
gpeacock@palm.com
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |553
application/vnd.pg.format Proprietary Proctor & Gamble Stan-
dard Reporting System.
str April Gandert
TN152
Procter & Gamble Way
Cincinnati, Ohio 45202
(513) 983-4249
application/vnd.pg.osasli Proprietary Proctor & Gamble Stan-
dard Reporting System.
ei6 Same as above
application/vnd.
powerbuilder6
application/vnd.
powerbuilder6-s
application/vnd.
powerbuilder7
application/vnd.
powerbuilder7-s
application/vnd.
powerbuilder75
application/vnd.
powerbuilder75-s
Used only by Sybase PowerBuilder
release 6, 7, and 7.5 runtime environ-
ments, nonsecure and secure.
pbd Reed Shilts
reed.shilts@sybase.com
application/vnd.
previewsystems.box
Preview Systems ZipLock/VBox
product.
box,
vbox
Roman Smolgovsky
romans@previewsystems.com
http //www.previewsystems.com
application/vnd.publishare-
delta-tree
Used by Capella Computers’
PubliShare runtime environment.
qps Oren Ben-Kiki
publishare-delta-tree@capella.co.il
application/vnd.rapid Emultek’s rapid packaged
applications.
zrp Itay Szekely
etay@emultek.co.il
application/vnd.s3sms Integratesthe transfer mechanisms of
the Sonera SmartTrust products into
the Internet infrastructure.
Lauri Tarkkala
Lauri.Tarkkala@sonera.com
http //www.smarttrust.com
application/vnd.seemail Supports the transmission of SeeMail
files. SeeMail is an application that
captures video and sound and uses
bitwise compression to compress and
archive the two pieces into one file.
see Steven Webb
steve@wynde.com
http //www.realmediainc.com
application/vnd.shana.
informed.formdata
Shana e-forms data formats. ifm Guy Selzler
Shana Corporation
gselzler@shana.com
application/vnd.shana.
informed.formtemp
Shana e-forms data formats. itp Same as above
application/vnd.shana.
informed.interchange
Shana e-forms data formats. iif, iif1 Same as above
application/vnd.shana.
informed.package
Shana e-forms data formats. ipk, ipkg Same as above
application/vnd.street-stream Proprietary to Street Technologies. Glenn Levitt
Street Technologies
streetd1@ix.netcom.com
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
554 |Appendix D: MIME Types
application/vnd.svd Dateware Electronics SVD files. Scott Becker
dataware@compumedia.com
application/vnd.swiftview-ics Supports SwiftView®. Randy Prakken
tech@ndg.com
http //www.ndg.com/svm htm
application/vnd.triscape.mxs Supports Triscape Map Explorer. mxs Steven Simonoff
scs@triscape.com
application/vnd.trueapp True BASIC files. tra J. Scott Hepler
scott@truebasic.com
application/vnd.truedoc Proprietary to Bitstream, Inc. Brad Chase
brad_chase@bitstream.com
application/vnd.ufdl UWI’s UFDL files. ufdl, ufd,
frm
Dave Manning
dmanning@uwi.com
http //www.uwi.com/
application/vnd.uplanet.alert
application/vnd.uplanet.
alert-wbxml
application/vnd.uplanet.
bearer-choi-wbxml
application/vnd.uplanet.
bearer-choice
application/vnd.uplanet.
cacheop
application/vnd.uplanet.
cacheop-wbxml
application/vnd.uplanet.
channel
application/vnd.uplanet.
channel-wbxml
application/vnd.uplanet.list
application/vnd.uplanet.list-
wbxml
application/vnd.uplanet.
listcmd
application/vnd.uplanet.
listcmd-wbxml
application/vnd.uplanet.
signal
Formatsused by Unwired Planet (now
Openwave) UP browser micro-
browser for mobile devices.
iana-registrar@uplanet.com
http //www.openwave.com
application/vnd.vcx VirtualCatalog. vcx Taisuke Sugimoto
sugimototi@noanet.nttdata.co.jp
application/vnd.vectorworks VectorWorks graphics files. mcd Paul C. Pharr
pharr@diehlgraphsoft.com
application/vnd.vidsoft.
vidconference
VidConference format. vsc Robert Hess
hess@vidsoft.de
application/vnd.visio Visio files. vsd, vst,
vsw, vss
Troy Sandal
troys@visio.com
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |555
application/vnd.vividence.
scriptfile
Vividence files. vsf, vtd,
vd
Mark Risher
markr@vividence.com
application/vnd.wap.sic WAP Service Indication format. sic,
wbxml
WAP Forum Ltd
http //www.wapforum.org
application/vnd.wap.slc WAP Service Loading format.
Anything that conforms to the Service
Loading specification, available at
http://www.wapforum.org.
slc,
wbxml
Same as above
application/vnd.wap.wbxml WAP WBXML binary XML format for
wireless devices.
wbxml Same as above
“WAP Binary XML Content
Format—WBXML version 1.1”
application/vnd.wap.wmlc WAP WML format for wireless
devices.
wmlc,
wbxml
Same as above
application/vnd.wap.
wmlscriptc
WAP WMLScript format. wmlsc Same as above
application/vnd.webturbo WebTurbo format. wtb Yaser Rehem
Sapient Corporation
yrehem@sapient.com
application/vnd.wrq-hp3000-
labelled
Supports HP3000 formats. support@wrq.com
support@3k.com
application/vnd.wt.stf Supports Worldtalk software. stf Bill Wohler
wohler@worldtalk.com
application/vnd.xara Xara files are saved by CorelXARA, an
object-oriented vector graphics pack-
age written by Xara Limited (and
marketed by Corel).
xar David Matthewman
david@xara.com
http //www.xara.com
application/vnd.xfdl UWI’s XFDL files. xfdl, xfd,
frm
Dave Manning
dmanning@uwi.com
http //www.uwi.com
application/vnd.yellowriver-
custom-menu
Supports the Yellow River Custom-
Menu plug-in, which provides cus-
tomized browser drop-down menus.
cmp yellowriversw@yahoo.com
application/whoispp-query Defines Whois++ protocol queries
within MIME.
RFC 2957
application/whoispp-
response
Defines Whois++ protocol responses
within MIME.
RFC 2958
application/wita Wang Information Transfer
Architecture.
Document number 715-0050A,
Wang Laboratories
campbell@redsox.bsw.com
application/wordperfect5.1 WordPerfect documents.
application/x400-bp Carries any X.400 body part for which
there is no registered IANA mapping.
RFC 1494
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
556 |Appendix D: MIME Types
application/x-bcpio Old-style binary CPIO archives. bcpio
application/x-cdlink Allows integration of CD-ROM media
within web pages.
vcd http //www.cdlink.com
application/x-chess-pgm From Apache mime.types. pgn
application/x-compress Binary data from Unix compress. z
application/x-cpio CPIO archive file. cpio
application/x-csh CSH scripts. csh
application/x-director Macromedia director files. dcr, dir,
dxr
application/x-dvi TeX DVI files. dvi
application/x-futuresplash From Apache mime.types. spl
application/x-gtar GNU tar archives. gtar
application/x-gzip GZIP compressed data. gz
application/x-hdf From Apache mime.types. hdf
application/x-javascript JavaScript files. js
application/x-koan Supports the automatic playback of
Koan music files over the Internet, by
helper applications such as SSEYO
Koan Netscape Plugin.
skp, skd,
skt, skm
application/x-latex LaTeX files. latex
application/x-netcdf NETCDF files. nc, cdf
application/x-sh SH scripts. sh
application/x-shar SHAR archives. shar
application/x-shockwave-
flash
Macromedia Flash files. swf
application/x-stuffit StuffIt archives. sit
application/x-sv4cpio Unix SysV R4 CPIO archives. sv4cpio
application/x-sv4crc Unix SysV R4 CPIO w/CRC archives. sv4crc
application/x-tar TAR archives. tar
application/x-tcl TCL scripts. tcl
application/x-tex TeX files. tex
application/x-texinfo TeX info files. texinfo,
texi
application/x-troff TROFF files. t, tr, roff
application/x-troff-man TROFF Unix manpages. man
application/x-troff-me TROFF+me files. me
application/x-troff-ms TROFF+ms files ms
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |557
audio/*
Table D-4 summarizes audio content types.
application/x-ustar The extended tar interchange format. ustar See the IEEE 1003.1(1990)
specifications
application/x-wais-source WAIS source structure. src
application/xml Extensible Markup Language format
file (use text/xml if you want the file
treated as plain text by browsers, etc.).
xml, dtd RFC 2376
application/zip PKWARE zip archives. zip
Table D-4. “Audio” MIME types
MIME type Description Extension Contact and reference
audio/32kadpcm 8 kHz ADPCM audio encoding. RFC 2421
audio/basic Audio encoded with 8-kHz monaural
8-bit ISDN u-law PCM.
au, snd RFC 1341
audio/G.772.1 G.722.1compresses 50Hz–7kHzaudio
signals into 24 kbit/s or 32 kbit/s. It
may be used for speech, music, and
other types of audio.
RFC 3047
audio/L16 Audio/L16 is based on L16, described
in RFC 1890. L16 denotes uncom-
pressed audio data, using 16-bit
signed representation.
RFC 2586
audio/MP4A-LATM MPEG-4 audio. RFC 3016
audio/midi MIDI music files. mid,
midi, kar
audio/mpeg MPEG encoded audio files. mpga,
mp2,
mp3
RFC 3003
audio/parityfec Parity-based forward error correction
for RTP audio.
RFC 3009
audio/prs.sid Commodore 64 SID audio files. sid, psid http //www.geocities.com/
SiliconValley/Lakes/5147/sidplay/
docs.html#fileformats
audio/telephone-event Logical telephone event. RFC 2833
audio/tone Telephonic sound pattern. RFC 2833
audio/vnd.cns.anp1 Supports voice and unified messaging
application features available on the
Access NP network services platform
from Comverse Network Systems.
Ann McLaughlin
Comverse Network Systems
amclaughlin@comversens.com
Table D-3. “Application” MIME types (continued)
MIME type Description Extension Contact and reference
558 |Appendix D: MIME Types
audio/vnd.cns.inf1 Supports voice and unified messaging
application features available on the
TRILOGUE Infinity network services
platform from Comverse Network
Systems.
Same as above
audio/vnd.digital-winds Digital Winds music is never-ending,
reproducible, and interactive MIDI
music in very small packages (<3K).
eol Armands Strazds
armands.strazds@medienhaus-
bremen.de
audio/vnd.everad.plj Proprietary EverAD audio encoding. plj Tomer Weisberg
tomer@everad.com
audio/vnd.lucent.voice Voice messaging including Lucent
Technologies’ Intuity™ AUDIX® Mul-
timedia Messaging System and the
Lucent Voice Player.
lvp Frederick Block
rickblock@lucent.com
http //www.lucent.com/lvp/
audio/vnd.nortel.vbk Proprietary Nortel Networks Voice
Block audio encoding.
vbk Glenn Parsons
Glenn.Parsons@NortelNetworks.com
audio/vnd.nuera.ecelp4800 Proprietary Nuera Communications
audio and speech encoding, available
in Nuera voice-over-IP gateways, ter-
minals, application servers, and as a
media service for various host plat-
forms and OSs.
ecelp4800 Michael Fox
mfox@nuera.com
audio/vnd.nuera.ecelp7470 Same as above. ecelp7470 Same as above
audio/vnd.nuera.ecelp9600 Same as above. ecelp9600 Same as above
audio/vnd.octel.sbc Variable-rate encoding averaging 18
kbps used for voice messaging in
Lucent Technologies’ Sierra™, Over-
ture™, and IMA™ platforms.
Jeff Bouis
jbouis@lucent.com
audio/vnd.qcelp Qualcomm audio encoding. qcp Andy Dejaco
adejaco@qualcomm.com
audio/vnd.rhetorex.
32kadpcm
32-kbps Rhetorex™ ADPCM audio
encoding used in voice messaging
products such as Lucent Technolo-
gies’s CallPerformer™, Unified Mes-
senger™, and other products.
Jeff Bouis
jbouis@lucent.com
audio/vnd.vmx.cvsd Audio encoding used in voice messag-
ing products including Lucent Tech-
nologies’ Overture200™, Overture
300™, and VMX 300™ product lines.
Same as above
audio/x-aiff AIFF audio file format. aif, aiff,
aifc
audio/x-pn-realaudio RealAudio metafile format by Real
Networks (formerly Progressive
Networks).
ram, rm
audio/x-pn-realaudio-plugin From Apache mime.types. rpm
Table D-4. “Audio” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |559
chemical/*
Much of the information in Table D-5 was obtained courtesy of the “Chemical
MIME Home Page” (http://www.ch.ic.ac.uk/chemime/).
audio/x-realaudio RealAudio audio format by Real
Networks (formerly Progressive
Networks).
ra
audio/x-wav WAV audio files. wav
Table D-5. “Chemical” MIME types
MIME type Description Extension Contact and reference
chemical/x-alchemy Alchemy format alc http //www.camsoft.com
chemical/x-cache-csf csf
chemical/x-cactvs-binary CACTVS binary format cbin http //cactvs.cit.nih.gov
chemical/x-cactvs-ascii CACTVS ASCII format cascii http //cactvs.cit.nih.gov
chemical/x-cactvs-table CACTVS table format ctab http //cactvs.cit.nih.gov
chemical/x-cdx ChemDraw eXchange file cdx http //www.camsoft.com
chemical/x-cerius MSI Cerius II format cer http //www.msi.com
chemical/x-chemdraw ChemDraw file chm http //www.camsoft.com
chemical/x-cif Crystallographic Interchange Format cif http //www.bernstein-plus-
sons.com/software/rasmol/
http //ndbserver.rutgers.edu/NDB/
mmcif/examples/index.html
chemical/x-mmcif MacroMolecular CIF mcif Same as above
chemical/x-chem3d Chem3D format c3d http //www.camsoft.com
chemical/x-cmdf CrystalMaker Data Format cmdf http //www.crystalmaker.co.uk
chemical/x-compass Compass program of the Takahashi cpa
chemical/x-crossfire Crossfire file bsd
chemical/x-cml Chemical Markup Language cml http //www.xml-cml.org
chemical/x-csml Chemical Style Markup Language csml,
csm
http //www.mdli.com
chemical/x-ctx Gasteiger group CTX file format ctx
chemical/x-cxf cxf
chemical/x-daylight-smiles Smiles format smi http //www.daylight.com/dayhtml/
smiles/index.html
chemical/x-embl-dl-
nucleotide
EMBL nucleotide format emb http //mercury.ebi.ac.uk
chemical/x-galactic-spc SPC format for spectral and
chromatographic data
spc http //www.galactic.com/galactic/
Data/spcvue.htm
Table D-4. “Audio” MIME types (continued)
MIME type Description Extension Contact and reference
560 |Appendix D: MIME Types
chemical/x-gamess-input GAMESS Input format inp, gam http //www.msg.ameslab.gov/
GAMESS/Graphics/
MacMolPlt.shtml
chemical/x-gaussian-input Gaussian Input format gau http //www.mdli.com
chemical/x-gaussian-
checkpoint
Gaussian Checkpoint format fch, fchk http //products.camsoft.com
chemical/x-gaussian-cube Gaussian Cube (Wavefunction) format cub http //www.mdli.com
chemical/x-gcg8-sequence gcg
chemical/x-genbank ToGenBank format gen
chemical/x-isostar IsoStar Library of intermolecular
interactions
istr, ist http //www.ccdc.cam.ac.uk
chemical/x-jcamp-dx JCAMP Spectroscopic Data Exchange
format
jdx, dx http //www.mdli.com
chemical/x-jjc-review-surface Re_View3 Orbital Contour files rv3 http //www.brunel.ac.uk/depts/
chem/ch241s/re_view/rv3 htm
chemical/x-jjc-review-xyz Re_View3 Animation files xyb http //www.brunel.ac.uk/depts/
chem/ch241s/re_view/rv3 htm
chemical/x-jjc-review-vib Re_View3 Vibration files rv2, vib http //www.brunel.ac.uk/depts/
chem/ch241s/re_view/rv3 htm
chemical/x-kinemage Kinetic (Protein Structure) Images kin http //www.faseb.org/protein/
kinemages/MageSoftware.html
chemical/x-macmolecule MacMolecule file format mcm
chemical/x-macromodel-
input
MacroModel Molecular Mechanics mmd,
mmod
http //www.columbia.edu/cu/
chemistry/
chemical/x-mdl-molfile MDL Molfile mol http //www.mdli.com
chemical/x-mdl-rdfile Reaction data file rd http //www.mdli.com
chemical/x-mdl-rxnfile MDL Reaction format rxn http //www.mdli.com
chemical/x-mdl-sdfile MDL Structure data file sd http //www.mdli.com
chemical/x-mdl-tgf MDL Transportable Graphics Format tgf http //www.mdli.com
chemical/x-mif mif
chemical/x-mol2 Portable representation of a SYBYL
molecule
mol2 http //www.tripos.com
chemical/x-molconn-Z Molconn-Z format b http //www.eslc.vabiotech.com/
molconn/molconnz.html
chemical/x-mopac-input MOPAC Input format mop http //www.mdli.com
chemical/x-mopac-graph MOPAC Graph format gpt http //products.camsoft.com
chemical/x-ncbi-asn1 asn (old
form)
chemical/x-ncbi-asn1-binary val
chemical/x-pdb Protein DataBank pdb pdb http //www.mdli.com
Table D-5. “Chemical” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |561
image/*
Table D-6 summarizes some of the image types commonly exchanged by email and
HTTP.
chemical/x-swissprot SWISS-PROT protein sequence
database
sw http //www.expasy.ch/spdbv/text/
download.htm
chemical/x-vamas-iso14976 Versailles Agreement on Materials
and Standards
vms http //www.acolyte.co.uk/JISO/
chemical/x-vmd Visual Molecular Dynamics vmd http //www.ks.uiuc.edu/Research/
vmd/
chemical/x-xtel Xtelplot file format xtel http //www.recipnet.indiana.edu/
graphics/xtelplot/xtelplot htm
chemical/x-xyz Co-ordinate Animation format xyz http //www.mdli.com
Table D-6. “Image” MIME types
MIME type Description Extension Contact and reference
image/bmp Windows BMP image format. bmp
image/cgm Computer Graphics Metafile (CGM) is
an International Standard for the por-
table storage and transfer of 2-D
illustrations.
Alan Francis
A.H.Francis@open.ac.uk
See ISO 8632:1992, IS 8632:1992
Amendment 1 (1994), and IS 8632:
1992 Amendment 2 (1995)
image/g3fax G3 Facsimile byte streams. RFC 1494
image/gif Compuserve GIF images. gif RFC 1341
image/ief ief RFC 1314
image/jpeg JPEG images. jpeg,jpg,
jpe, jfif
JPEG Draft Standard ISO 10918-1
CD
image/naplps North American Presentation Layer
Protocol Syntax (NAPLPS) images.
ANSI X3.110-1983 CSA T500-1983
image/png Portable Network Graphics (PNG)
images.
png Internet draft draft-boutell-png-
spec-04.txt, “Png (Portable
Network Graphics) Specification
Version 1.0”
image/prs.btif Format used by Nations Bank for BTIF
image viewing of checks and other
applications.
btif, btf Arthur Rubin
arthurr@crt.com
image/prs.pti PTI encoded images. pti Juern Laun
juern.laun@gmx.de
http //server.hvzgymn.wn.schule-
bw.de/pti/
image/tiff TIFF images. tiff, tif RFC 2302
Table D-5. “Chemical” MIME types (continued)
MIME type Description Extension Contact and reference
562 |Appendix D: MIME Types
image/vnd.cns.inf2 Supports application features avail-
ableon the TRILOGUE Infinity network
services platform from Comverse Net-
work Systems.
Ann McLaughlin
Comverse Network Systems
amclaughlin@comversens.com
image/vnd.dxf DXF vector CAD files. dxf
image/vnd.fastbidsheet A FastBid Sheet contains a raster or
vector image that represents an engi-
neering or architectural drawing.
fbs Scott Becker
scottb@bxwa.com
image/vnd.fpx Kodak FlashPix images. fpx Chris Wing
format_change_request@kodak.
com
http //www.kodak.com
image/vnd.fst Image format from FAST Search and
Transfer.
fst Arild Fuldseth
Arild.Fuldseth@fast.no
image/vnd.fujixerox.edmics-
mmr
Fuji Xerox EDMICS MMR image
format.
mmr Masanori Onda
Masanori.Onda@fujixerox.co.jp
image/vnd.fujixerox.edmics-
rlc
Fuji Xerox EDMICS RLC image format. rlc Same as above
image/vnd.mix MIX files contain binary data in
streams that are used to represent
images and related information. They
are used by Microsoft PhotDraw and
PictureIt software.
Saveen Reddy2
saveenr@microsoft.com
image/vnd.net-fpx Kodak FlashPix images. Chris Wing
format_change_request@kodak.
com
http //www.kodak.com
image/vnd.wap.wbmp From Apache mime.types. wbmp
image/vnd.xiff Extended Image Format used by Pagis
software.
xif Steve Martin
smartin@xis.xerox.com
image/x-cmu-raster From Apache mime.types. ras
image/x-portable-anymap PBM generic images. pnm Jeff Poskanzer
http //www.acme.com/software/
pbmplus/
image/x-portable-bitmap PBM bitmap images. pbm Same as above
image/x-portable-graymap PBM grayscale images. pgm Same as above
image/x-portable-pixmap PBM color images. ppm Same as above
image/x-rgb Silicon Graphics’s RGB images. rgb
image/x-xbitmap X-Window System bitmap images. xbm
image/x-xpixmap X-Window System color images. xpm
image/x-xwindowdump X-Window System screen capture
images.
xwd
Table D-6. “Image” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |563
message/*
Messages are composite types used to communicate data objects (through email,
HTTP, or other transport protocols). Table D-7 describes the common MIME mes-
sage types.
model/*
The model MIME type is an IETF-registered extension type. It represents mathemati-
cal models of physical worlds, for computer-aided design, and 3-D graphics.
Table D-8 describes some of the model formats.
Table D-7. “Message” MIME types
MIME type Description Extension Contact and reference
message/delivery-status
message/disposition-
notification
RFC 2298
message/external-body RFC 1341
message/http RFC 2616
message/news Defines a way to transmit news arti-
cles via email for human reading—
message/rfc822 is not sufficient
because news headers have seman-
tics beyond those defined by RFC 822.
RFC 1036
message/partial Permits the fragmented transmission
of bodies that are thought to be too
large to be sent directly by email.
RFC 1341
message/rfc822 A complete email message. RFC 1341
message/s-http Secure HTTP messages, an alternative
to HTTP over SSL.
RFC 2660
Table D-8. “Model” MIME types
MIME type Description Extension Contact and reference
model/iges The Initial Graphics Exchange Specifi-
cation (IGES) defines a neutral data
format that allows for the digital
exchange of information between
computer-aided design (CAD) systems.
igs, iges RFC 2077
model/mesh msh,
mesh,
silo
RFC 2077
model/vnd.dwf DWF CAD files. dwf Jason Pratt
jason.pratt@autodesk.com
model/vnd.flatland.3dml Supports 3DML models supported by
Flatland products.
3dml,
3dm
Michael Powers
pow@flatland.com
http //www.flatland.com
564 |Appendix D: MIME Types
multipart/*
Multipart MIME types are composite objects that contain other objects. The sub-
type describes the implementation of the multipart packaging and how to process
the components. Multipart media types are summarized in Table D-9.
model/vnd.gdl
model/vnd.gs-gdl
The Geometric Description Language
(GDL) is a parametric object definition
language for ArchiCAD by Graphisoft.
gdl,gsm,
win, dor,
lmp,rsm,
msm,
ism
Attila Babits
ababits@graphisoft.hu
http //www.graphisoft.com
model/vnd.gtw Gen-Trix models. gtw Yutaka Ozaki
yutaka_ozaki@gen.co.jp
model/vnd.mts MTS model format by Virtue. mts Boris Rabinovitch
boris@virtue3d.com
model/vnd.parasolid.trans-
mit.binary
Binary Parasolid modeling file. x_b http //www.ugsolutions.com/
products/parasolid/
model/vnd.parasolid.trans-
mit.text
Text Parasolid modeling file. x_t http //www.ugsolutions.com/
products/parasolid/
model/vnd.vtu VTU model format by Virtue. vtu Boris Rabinovitch
boris@virtue3d.com
model/vrml Virtual Reality Markup Language
format files.
wrl, vrml RFC 2077
Table D-9. “Multipart” MIME types
MIME type Description Extension Contact and reference
multipart/alternative The content consists of a list of alter-
native representations, each with its
own Content-Type. The client can
select the best supported component.
RFC 1341
multipart/appledouble Apple Macintosh files contain
“resource forks” and other desktop
data that describes the actual file con-
tents. This multipart content sends
the Apple metadata in one part and
the actual content in another part.
http //www.isi.edu/in-notes/iana/
assignments/media-types/
multipart/appledouble
multipart/byteranges When an HTTP message includes the
content of multiple ranges, these are
transmitted in a “multipart/byter-
anges” object. This media type
includes two or more parts, separated
by MIME boundaries, each with its
own Content-Type and Content-
Range fields.
RFC 2068
Table D-8. “Model” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |565
text/*
Text media types contain characters and potential formatting information. Table D-10
summarizes text MIME types.
multipart/digest Contains a collection of individual
email messages, in an easy-to-read
form.
RFC 1341
multipart/encrypted Uses two parts to support crypto-
graphically encrypted content. The
first part contains the control infor-
mation necessary to decrypt the data
in the second body part and is labeled
according to the value of the protocol
parameter. The second part contains
the encrypted data in type applica-
tion/octet-stream.
RFC 1847
multipart/form-data Used to bundle up a set of values as
the result of a user filling out a form.
RFC 2388
multipart/header-set Separates user data from arbitrary
descriptive metadata.
http //www.isi.edu/in-notes/iana/
assignments/media-types/
multipart/header-set
multipart/mixed A collection of objects. RFC 1341
multipart/parallel Syntactically identical to multipart/
mixed,butall of the parts are intended
to be presented simultaneously, on
systems capable of doing so.
RFC 1341
multipart/related Intended for compound objects con-
sisting of several interrelated body
parts. The relationships between the
body parts distinguish them from
other object types. These relation-
ships often are represented by links
internal to the object’s components
that reference the other components.
RFC 2387
multipart/report Defines a general container type for
electronic mail reports of any kind.
RFC 1892
multipart/signed Uses two parts to support crypto-
graphically signed content. The first
part is the content, including its MIME
headers. The second part contains the
information necessary to verify the
digital signature.
RFC 1847
multipart/voice-message Provides a mechanism for packaging
a voice message into one container
that is tagged as VPIM v2–compliant.
RFCs 2421 and 2423
Table D-9. “Multipart” MIME types (continued)
MIME type Description Extension Contact and reference
566 |Appendix D: MIME Types
Table D-10. “Text” MIME types
MIME type Description Extension Contact and reference
text/calendar Supports the iCalendar calendaring
and scheduling standard.
RFC 2445
text/css Cascading Style Sheets. css RFC 2318
text/directory Holds record data from a directory
database, such as LDAP.
RFC 2425
text/enriched Simple formatted text, supporting
fonts, colors, and spacing. SGML-like
tags are used to begin and end
formatting.
RFC 1896
text/html HTML file. html,
htm
RFC 2854
text/parityfec Forward error correction for text
streamed in an RTP stream.
RFC 3009
text/plain Plain old text. asc, txt
text/prs.lines.tag Supports tagged forms, as used for
email registration.
tag, dsc John Lines
john@paladin.demon.co.uk
http //www.paladin.demon.co.uk/
tag-types/
text/rfc822-headers Used to bundle a set of email headers,
such as when sending mail failure
reports.
RFC 1892
text/richtext Older form of enriched text. See text/
enriched.
rtx RFC 1341
text/rtf The Rich Text Format (RTF) is a
method of encoding formatted text
and graphics for transfer between
applications. The format is widely
supported by word-processing appli-
cations on the MS-DOS, Windows,
OS/2, and Macintosh platforms.
rtf
text/sgml SGML markup files. sgml,
sgm
RFC 1874
text/t140 Supports standardized T.140 text, as
used in synchronized RTP multimedia.
RFC 2793
text/tab-separated-values TSV is a popular method of data inter-
change among databases and spread-
sheets and word processors. It
consists of a set of lines, with fields
separated by tab characters.
tsv http //www.isi.edu/in-notes/iana/
assignments/media-types/text/tab-
separated-values
text/uri-list Simple, commented lists of URLs and
URNs used by URN resolvers, and any
other applications that need to com-
municate bulk URI lists.
uris, uri RFC 2483
MIME Type Tables |567
text/vnd.abc ABC files are a human-readable for-
mat for musical scores.
abc http //www.gre.ac.uk/
~c.walshaw/abc/
http //home1.swipnet.se/
~w-11382/abcbnf.htm
text/vnd.curl Provides a set of content definition
languages interpreted by the CURL
runtime plug-in.
curl Tim Hodge
thodge@curl.com
text/vnd.DMClientScript CommonDM Client Script files are
used as hyperlinks to non-http sites
(such as BYOND, IRC, or telnet)
accessed by the Dream Seeker client
application.
dms Dan Bradley
dan@dantom.com
http //www.byond.com/code/ref/
text/vnd.fly Fly is a text preprocessor that uses a
simple syntax to create an interface
between databases and web pages.
fly John-Mark Gurney
jmg@flyidea.com
http //www.flyidea.com
text/vnd.fmi.flexstor For use in the SUVDAMA and
UVRAPPF projects.
flx http //www.ozone.fmi.fi/
SUVDAMA/
http //www.ozone.fmi.fi/UVRAPPF/
text/vnd.in3d.3dml For In3D Player. 3dml,
3dm
Michael Powers
powers@insideout.net
text/vnd.in3d.spot For In3D Player. spot, spo Same as above
text/vnd.IPTC.NewsML NewsML format specified by the
International Press Telecommunica-
tions Council (IPTC).
xml David Allen
m_director_iptc@dial.pipex.com
http //www.iptc.org
text/vnd.IPTC.NITF NITF format specified by the IPTC. xml Same as above
http //www.nitf.org
text/vnd.latex-z Supports LaTeX documents contain-
ing Z notation. Z notation (pro-
nounced “zed”), is based on Zermelo-
Fraenkel set theory and first order
predicate logic, and it is useful for
describing computer systems.
http //www.comlab.ox.ac.uk/
archive/z/
text/vnd.motorola.reflex Provides a common method for sub-
mitting simple text messages from
ReFLEX™ wireless devices.
Mark Patton
fmp014@email.mot.com
Part of the FLEXsuite™ of Enabling
Protocols specification available
from Motorola under the licensing
agreement
text/vnd.ms-mediapackage This type is intended to be handled by
the Microsoft application programs
MStore.exe and 7 storDB.exe.
mpf Jan Nelson
jann@microsoft.com
Table D-10. “Text” MIME types (continued)
MIME type Description Extension Contact and reference
568 |Appendix D: MIME Types
video/*
Table D-11 lists some popular video movie formats. Note that some video formats
are classified as application types.
text/vnd.wap.si Service Indication (SI) objects contain
a message describing an event and a
URI describing where to load the cor-
responding service.
si, xml WAP Forum Ltd
http //www.wapforum.org
text/vnd.wap.sl The Service Loading (SL) content type
provides a means to convey a URI to a
user agent in a mobile client. The cli-
ent itself automatically loads the
content indicated by that URI and
executes it in the addressed user
agent without user intervention
when appropriate.
sl, xml Same as above
text/vnd.wap.wml Wireless Markup Language (WML) is
a markup language, based on XML,
that defines content and user inter-
face for narrow-band devices, includ-
ing cellular phones and pagers.
wml Same as above
text/vnd.wap.wmlscript WMLScript is an evolution of Java-
Script for wireless devices.
wmls Same as above
text/x-setext From Apache mime.types. etx
text/xml Extensible Markup Language format
file (use application/xml if you want
the browser to save to file when
downloaded).
xml RFC 2376
Table D-11. “Video” MIME types
MIME type Description Extension Contact and reference
video/MP4V-ES MPEG-4 video payload, as carried by
RTP.
RFC 3016
video/mpeg Video encoded per the ISO 11172 CD
MPEG standard.
mpeg,
mpg,
mpe
RFC 1341
video/parityfec Forward error correcting video format
for data carried through RTP streams.
RFC 3009
video/pointer Transporting pointer position infor-
mation for presentations.
RFC 2862
video/quicktime Apple Quicktime video format. qt, mov http //www.apple.com
video/vnd.fvt Video format from FAST Search &
Transfer.
fvt Arild Fuldseth
Arild.Fuldseth@fast.no
Table D-10. “Text” MIME types (continued)
MIME type Description Extension Contact and reference
MIME Type Tables |569
Experimental Types
The set of primary types supports most content types. Table D-12 lists one experi-
mental type, for conferencing software, that is configured in some web servers.
video/vnd.motorola.video
video/vnd.motorola.videop
Proprietary formats used by products
from Motorola ISG.
Tom McGinty
Motorola ISG
tmcginty@dma.isg.mot
video/vnd.mpegurl This media type consists of a series of
URLs of MPEG Video files.
mxu Heiko Recktenwald
uzs106@uni-bonn.de
“Power and Responsibility: Conver-
sations with Contributors,”Guy van
Belle, et al., LMJ 9 (1999), 127–
133, 129 (MIT Press)
video/vnd.nokia.interleaved-
multimedia
Used in Nokia 9210 Communicator
video player and related tools.
nim Petteri Kangaslampi
petteri.kangaslampi@nokia.com
video/x-msvideo Microsoft AVI movies. avi http //www.microsoft.com
video/x-sgi-movie Silicon Graphics’s movie format. movie http //www.sgi.com
Table D-12. Extension MIME types
MIME type Description Extension Contact and reference
x-conference/x-cooltalk Collaboration tool from Netscape ice
Table D-11. “Video” MIME types (continued)
MIME type Description Extension Contact and reference
570
APPENDIX E
Base-64 Encoding
Base-64 encoding is used by HTTP, for basic and digest authentication, and by sev-
eral HTTP extensions. This appendix explains base-64 encoding and provides con-
version tables and pointers to Perl software to help you correctly use base-64
encoding in HTTP software.
Base-64 Encoding Makes Binary Data Safe
The base-64 encoding converts a series of arbitrary bytes into a longer sequence of
common text characters that are all legal header field values. Base-64 encoding lets
us take user input or binary data, pack it into a safe format, and ship it as HTTP
header field values without fear of them containing colons, newlines, or binary val-
ues that would break HTTP parsers.
Base-64 encoding was developed as part of the MIME multimedia electronic mail
standard, so MIME could transport rich text and arbitrary binary data between differ-
ent legacy email gateways.*Base-64 encoding is similar in spirit, but more efficient in
space, to the uuencode and BinHex standards for textifying binary data. Section 6.8
of MIME RFC 2045 details the base-64 algorithm.
Eight Bits to Six Bits
Base-64 encoding takes a sequence of 8-bit bytes, breaks the sequence into 6-bit
pieces, and assigns each 6-bit piece to one of 64 characters comprising the base-64
alphabet. The 64 possible output characters are common and safe to place in HTTP
header fields. The 64 characters include upper- and lowercase letters, numbers, +,
* Some mail gateways would silently strip many “non-printing” characters with ASCII values between 0 and
31. Other programs would interpret some bytes as flow control characters or other special control charac-
ters, or convert carriage returns to line feeds and the like. Some programs would experience fatal errors upon
receiving international characters with a value above 127 because the software was not “8-bit clean.”
Eight Bits to Six Bits |571
and /. The special character = also is used. The base-64 alphabet is shown in
Table E-1.
Note that because the base-64 encoding uses 8-bit characters to represent 6 bits of
information, base 64–encoded strings are about 33% larger than the original values.
Figure E-1 shows a simple example of base-64 encoding. Here, the three-character
input value “Ow!” is base 64–encoded, resulting in the four-character base 64–
encoded value “T3ch”. It works like this:
1. The string “Ow!” is broken into 3 8-bit bytes (0x4F, 0x77, 0x21).
2. The 3 bytes create the 24-bit binary value 010011110111011100100001.
3. These bits are segmented into the 6-bit sequences 010011, 110111, 01110,
100001.
4. Each of these 6-bit values represents a number from 0 to 63, corresponding to
one of 64 characters in the base-64 alphabet. The resulting base 64–encoded
string is the 4-character string “T3ch”, which can then be sent across the wire as
“safe” 8-bit characters, because only the most portable characters are used (let-
ters, numbers, etc.).
Table E-1. Base-64 alphabet
0 A 8 I 16 Q 24 Y 32 g 40 o 48 w 56 4
1 B 9 J 17 R 25 Z 33 h 41 p 49 x 57 5
2 C 10 K 18 S 26 a 34 i 42 q 50 y 58 6
3 D 11 L 19 T 27 b 35 j 43 r 51 z 59 7
4 E 12 M 20 U 28 c 36 k 44 s 52 0 60 8
5 F 13 N 21 V 29 d 37 l 45 t 53 1 61 9
6 G 14 O 22 W 30 e 38 m 46 u 54 2 62 +
7 H 15 P 23 X 31 f 39 n 47 v 55 3 63 /
Figure E-1. Base-64 encoding example
8-bit characters O
8-bit value (hexidecimal) $4F
8-bit value (binary) 010011110111011100100001
6-bit value (decimal) 19
Base-64 character T
w!
$77 $21
55 28 33
3ch
572 |Appendix E: Base-64 Encoding
Base-64 Padding
Base-64 encoding takes a sequence of 8-bit bytes and segments the bit stream into 6-
bit chunks. It is unlikely that the sequence of bits will divide evenly into 6-bit pieces.
When the bit sequence does not divide evenly into 6-bit pieces, the bit sequence is
padded with zero bits at the end to make the length of the bit sequence a multiple of
24 (the least common multiple of 6 and 8 bits).
When encoding the padded bit string, any group of 6 bits that is completely padding
(containing no bits from the original data) is represented by a special 65th symbol:
“=”. If a group of 6 bits is partially padded, the padding bits are set to zero.
Table E-2 shows examples of padding. The initial input string “a:a” is 3 bytes long,
or 24 bits. 24 is a multiple of 6 and 8, so no padding is required. The resulting base
64–encoded string is “YTph”.
However, when another character is added, the input string grows to 32 bits long.
The next smallest multiple of 6 and 8 is 48 bits, so 16 bits of padding are added. The
first 4 bits of padding are mixed with data bits. The resulting 6-bit group, 01xxxx, is
treated as 010000, 16 decimal, or base-64 encoding Q. The remaining two 6-bit
groups are all padding and are represented by “=”.
Perl Implementation
MIME::Base64 is a Perl module for base-64 encoding and decoding. You can read
about this module at http://www.perldoc.com/perl5.6.1/lib/MIME/Base64.html.
You can encode and decode strings using the MIME::Base64 encode_base64 and
decode_base64 methods:
use MIME::Base64;
$encoded = encode_base64('Aladdin:open sesame');
$decoded = decode_base64($encoded);
Table E-2. Base-64 padding examples
Input data Binary sequence (padding noted as “x”) Encoded data
a:a 011000 010011 101001 100001 YTph
a:aa 011000 010011 101001 100001 011000 01xxxx xxxxxx xxxxxx YTphYQ==
a:aaa 011000 010011 101001 100001 011000 010110 0001xx xxxxxx YTphYWE=
a:aaaa 011000 010011 101001 100001 011000 010110 000101 100001 YTphYWFh
For More Information |573
For More Information
For more information on base-64 encoding, see:
http://www.ietf.org/rfc/rfc2045.txt
Section 6.8 of RFC 2045, “MIME Part 1: Format of Internet Message Bodies,”
provides an official specification of base-64 encoding.
http://www.perldoc.com/perl5.6.1/lib/MIME/Base64.html
This web site contains documentation for the MIME::Base64 Perl module that
provides encoding and decoding of base-64 strings.
574
APPENDIX F
Digest Authentication
This appendix contains supporting data and source code for implementing HTTP
digest authentication facilities.
Digest WWW-Authenticate Directives
WWW-Authenticate directives are described in Table F-1, paraphrased from the
descriptions in RFC 2617. As always, refer to the official specifications for the most
up-to-date details.
Table F-1. Digest WWW-Authenticate header directives (from RFC 2617)
Directive Description
realm A string to be displayed to users so they know which username and password to use. This string should
contain at least the name of the host performing the authentication and might additionally indicate the
collection of users who might have access. An example might be “registered_users@gotham.news.com”.
nonce A server-specified data string that should be uniquely generated each time a 401 response is made. It is
recommended that this string be base-64 or hexadecimal data. Specifically, because the string is passed in
the header lines as a quoted string, the double-quote character is not allowed.
The contents of the nonce are implementation-dependent. The quality of the implementation depends on
a good choice. A nonce might, for example, be constructed as the base-64 encoding of:
time-stamp H(time-stamp ":" ETag ":" private-key)
where time-stamp is a server-generated time or other nonrepeating value, ETag is the value of the HTTP
ETag header associated with the requested entity, and private-key is data known only to the server. With a
nonce of this form, a server would recalculate the hash portion after receiving the client Authentication
header and reject the request if it did not match the nonce from that header or if the time-stamp value is
not recent enough. In this way, the server can limit the time of the nonce’s validity. The inclusion of the
ETag prevents a replay request for an updated version of the resource. (Note: including the IP address of
the client in the nonce appears to offer the server the ability to limit the reuse of the nonce to the same cli-
ent that originally got it. However, that would break proxy farms, where requests from a single user often
go through different proxies in the farm. Also, IP address spoofing is not that hard.)
An implementation might choose not to accept a previously used nonce or a previously used digest, to
protect against replay attacks, or it might choose to use one-time nonces or digests for POST or PUT
requests and time-stamps for GET requests.
Digest Authorization Directives |575
Digest Authorization Directives
Each of the Authorization directives is described in Table F-2, paraphrased from the
descriptions in RFC 2617. Refer to the official specifications for the most up-to-date
details.
domain A quoted, space-separated list of URIs (as specified in RFC 2396, “Uniform Resource Identifiers: Generic
Syntax”) that define the protection space. If a URI is an abs_path, it is relative to the canonical root URL of
the server being accessed. An absolute URI in this list may refer to a different server than the one being
accessed.
The client can use this list to determine the set of URIs for which the same authentication information may
be sent: any URI that has a URI in this list as a prefix (after both have been made absolute) may be
assumed to be in the same protection space.
If this directive is omitted or its value is empty, the client should assume that the protection space consists
of all URIs on the responding server.
This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is always
the entire proxy; if present, it should be ignored.
opaque A string of data, specified by the server, that should be returned by the client unchanged in the Authoriza-
tion header of subsequent requests with URIs in the same protection space. It is recommended that this
string be base-64 or hexadecimal data.
stale A flag indicating that the previous request from the client was rejected because the nonce value was stale.
If stale is TRUE (case-insensitive), the client may want to retry the request with a new encrypted response,
without reprompting the user for a new username and password. The server should set stale to TRUE only
if it receives a request for which the nonce is invalid but has a valid digest (indicating that the client knows
the correct username/password). If stale is FALSE, or anything other than TRUE, or the stale directive is not
present, the username and/or password are invalid, and new values must be obtained.
algorithm A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present, it
is assumed to be “MD5”. If the algorithm is not understood, the challenge should be ignored (and a differ-
ent one used, if there is more than one).
In this document, the string obtained by applying the digest algorithm to the data “data” with secret
“secret” will be denoted by “KD(secret, data)”, and the string obtained by applying the checksum algo-
rithm to the data “data”will be denoted “H(data)”. The notation “unq(X)”means the value of the quoted
string “X” without the surrounding quotes.
For the MD5 and MD5-sess algorithms:
H(data) = MD5(data)
HD(secret, data) = H(concat(secret, ":", data))
I.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The MD5-
sess algorithm is intended to allow efficient third-party authentication servers.
qop This directive is optional but is made so only for backward compatibility with RFC 2069 [6]; it should be
used by all implementations compliant with this version of the digest scheme.
If present, it is a quoted string of one or more tokens indicating the “quality of protection” values sup-
ported by the server. The value “auth”indicates authentication; the value “auth-int”indicates authentica-
tion with integrity protection. Unrecognized options must be ignored.
<extension> This directive allows for future extensions. Any unrecognized directives must be ignored.
Table F-1. Digest WWW-Authenticate header directives (from RFC 2617) (continued)
Directive Description
576 |Appendix F: Digest Authentication
Digest Authentication-Info Directives
Each of the Authentication-Info directives is described in Table F-3, paraphrased
from the descriptions in RFC 2617. Refer to the official specifications for the most
up-to-date details.
Table F-2. Digest Authorization header directives (from RFC 2617)
Directive Description
username The user’s name in the specified realm.
realm The realm passed to the client in the WWW-Authenticate header.
nonce The same nonce passed to the client in the WWW-Authenticate header.
uri The URI from the request URI of the request line; duplicated because proxies are allowed to change the
request line in transit, and we may need the original URI for proper digest verification calculations.
response This is the actual digest—the whole point of digest authentication! The response is a string of 32 hexadec-
imal digits, computed by a negotiated digest algorithm, which proves that the user knows the password.
algorithm A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present, it
is assumed to be “MD5”.
opaque A string of data, specified by the server in a WWW-Authenticate header, that should be returned by the
client unchanged in the Authorization header of subsequent requests with URIs in the same protection
space.
cnonce This must be specified if a qop directive is sent and must not be specified if the server did not send a qop
directive in the WWW-Authenticate header field.
The cnonce value is an opaque quoted string value provided by the client and used by both client and
server to avoid chosen plaintext attacks, to provide mutual authentication, and to provide some message-
integrity protection.
See the descriptions of the response-digest and request-digest calculations later in this appendix.
qop Indicates what “quality of protection” the client has applied to the message. If present, its value must be
one of the alternatives the server indicated it supports in the WWW-Authenticate header. These values
affect the computation of the request digest.
This is a single token, not a quoted list of alternatives, as in WWW-Authenticate.
This directive is optional, to preserve backward compatibility with a minimal implementation of RFC
2069, but it should be used if the server indicated that qop is supported by providing a qop directive in the
WWW-Authenticate header field.
nc This must be specified if a qop directive is sent and must not be specified if the server did not send a qop
directive in the WWW-Authenticate header field.
The value is the hexadecimal count of the number of requests (including the current request) that the cli-
ent has sent with the nonce value in this request. For example, in the first request sent in response to a
given nonce value, the client sends nc=“00000001”.
The purpose of this directive is to allow the server to detect request replays by maintaining its own copy of
this count—if the same nc value is seen twice, the request is a replay.
<extension> This directive allows for future extensions. Any unrecognized directive must be ignored.
Reference Code |577
Reference Code
The following code implements the calculations of H(A1), H(A2), request-digest, and
response-digest, from RFC 2617. It uses the MD5 implementation from RFC 1321.
File “digcalc.h”
#define HASHLEN 16
typedef char HASH[HASHLEN];
#define HASHHEXLEN 32
typedef char HASHHEX[HASHHEXLEN+1];
#define IN
Table F-3. Digest Authentication-Info header directives (from RFC 2617)
Directive Description
nextnonce The value of the nextnonce directive is the nonce the server wants the client to use for a future authenti-
cation response. The server may send the Authentication-Info header with a nextnonce field as a means of
implementing one-time or otherwise changing nonces. If the nextnonce field is present the client should
use it when constructing the Authorization header for its next request. Failure of the client to do so may
result in a reauthentication request from the server with “stale=TRUE”.
Server implementations should carefully consider the performance implications of the use of this mecha-
nism; pipelined requests will not be possible if every response includes a nextnonce directive that must be
used on the next request received by the server. Consideration should be given to the performance versus
security trade-offs of allowing an old nonce value to be used for a limited time to permit request pipelin-
ing. Use of the nonce count can retain most of the security advantages of a new server nonce without the
deleterious effects on pipelining.
qop Indicates the “quality of protection” options applied to the response by the server. The value “auth” indi-
cates authentication; the value “auth-int” indicates authentication with integrity protection. The server
should use the same value for the qop directive in the response as was sent by the client in the corre-
sponding request.
rspauth The optional response digest in the “response auth” directive supports mutual authentication—the
server proves that it knows the user’s secret, and, with qop=“auth-int”, it also provides limited integrity
protection of the response. The “response-digest” value is calculated as for the “request-digest” in the
Authorization header, except that if qop=“auth” or qop is not specified in the Authorization header for
the request, A2 is:
A2 = ":" digest-uri-value
and if qop=“auth-int”, A2 is:
A2 = ":" digest-uri-value ":" H(entity-body)
where digest-uri-value is the value of the uri directive on the Authorization header in the request. The
cnonce and nc values must be the same as the ones in the client request to which this message is a
response. The rspauth directive must be present if qop=“auth” or qop=“auth-int” is specified.
cnonce The cnonce value must be the same as the one in the client request to which this message is a response.
The cnonce directive must be present if qop=“auth” or qop=“auth-int” is specified.
nc The nc value must be the same as the one in the client request to which this message is a response. The nc
directive must be present if qop=“auth” or qop=“auth-int” is specified.
<extension> This directive allows for future extensions. Any unrecognized directive must be ignored.
578 |Appendix F: Digest Authentication
#define OUT
/* calculate H(A1) as per HTTP Digest spec */
void DigestCalcHA1(
IN char * pszAlg,
IN char * pszUserName,
IN char * pszRealm,
IN char * pszPassword,
IN char * pszNonce,
IN char * pszCNonce,
OUT HASHHEX SessionKey
);
/* calculate request-digest/response-digest as per HTTP Digest spec */
void DigestCalcResponse(
IN HASHHEX HA1, /* H(A1) */
IN char * pszNonce, /* nonce from server */
IN char * pszNonceCount, /* 8 hex digits */
IN char * pszCNonce, /* client nonce */
IN char * pszQop, /* qop-value: "", "auth", "auth-int" */
IN char * pszMethod, /* method from the request */
IN char * pszDigestUri, /* requested URL */
IN HASHHEX HEntity, /* H(entity body) if qop="auth-int" */
OUT HASHHEX Response /* request-digest or response-digest */
);
File “digcalc.c”
#include <global.h>
#include <md5.h>
#include <string.h>
#include "digcalc.h"
void CvtHex(
IN HASH Bin,
OUT HASHHEX Hex
)
{
unsigned short i;
unsigned char j;
for (i = 0; i < HASHLEN; i++) {
j = (Bin[i] >> 4) & 0xf;
if (j <= 9)
Hex[i*2] = (j + '0');
else
Hex[i*2] = (j + 'a' - 10);
j = Bin[i] & 0xf;
if (j <= 9)
Hex[i*2+1] = (j + '0');
else
Hex[i*2+1] = (j + 'a' - 10);
};
Hex[HASHHEXLEN] = '\0';
};
Reference Code |579
/* calculate H(A1) as per spec */
void DigestCalcHA1(
IN char * pszAlg,
IN char * pszUserName,
IN char * pszRealm,
IN char * pszPassword,
IN char * pszNonce,
IN char * pszCNonce,
OUT HASHHEX SessionKey
)
{
MD5_CTX Md5Ctx;
HASH HA1;
MD5Init(&Md5Ctx);
MD5Update(&Md5Ctx, pszUserName, strlen(pszUserName));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszRealm, strlen(pszRealm));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszPassword, strlen(pszPassword));
MD5Final(HA1, &Md5Ctx);
if (stricmp(pszAlg, "md5-sess") == 0) {
MD5Init(&Md5Ctx);
MD5Update(&Md5Ctx, HA1, HASHLEN);
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszNonce, strlen(pszNonce));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszCNonce, strlen(pszCNonce));
MD5Final(HA1, &Md5Ctx);
};
CvtHex(HA1, SessionKey);
};
/* calculate request-digest/response-digest as per HTTP Digest spec */
void DigestCalcResponse(
IN HASHHEX HA1, /* H(A1) */
IN char * pszNonce, /* nonce from server */
IN char * pszNonceCount, /* 8 hex digits */
IN char * pszCNonce, /* client nonce */
IN char * pszQop, /* qop-value: "", "auth", "auth-int" */
IN char * pszMethod, /* method from the request */
IN char * pszDigestUri, /* requested URL */
IN HASHHEX HEntity, /* H(entity body) if qop="auth-int" */
OUT HASHHEX Response /* request-digest or response-digest */
)
{
MD5_CTX Md5Ctx;
HASH HA2;
HASH RespHash;
HASHHEX HA2Hex;
// calculate H(A2)
MD5Init(&Md5Ctx);
MD5Update(&Md5Ctx, pszMethod, strlen(pszMethod));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszDigestUri, strlen(pszDigestUri));
if (stricmp(pszQop, "auth-int") == 0) {
580 |Appendix F: Digest Authentication
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, HEntity, HASHHEXLEN);
};
MD5Final(HA2, &Md5Ctx);
CvtHex(HA2, HA2Hex);
// calculate response
MD5Init(&Md5Ctx);
MD5Update(&Md5Ctx, HA1, HASHHEXLEN);
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszNonce, strlen(pszNonce));
MD5Update(&Md5Ctx, ":", 1);
if (*pszQop) {
MD5Update(&Md5Ctx, pszNonceCount, strlen(pszNonceCount));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszCNonce, strlen(pszCNonce));
MD5Update(&Md5Ctx, ":", 1);
MD5Update(&Md5Ctx, pszQop, strlen(pszQop));
MD5Update(&Md5Ctx, ":", 1);
};
MD5Update(&Md5Ctx, HA2Hex, HASHHEXLEN);
MD5Final(RespHash, &Md5Ctx);
CvtHex(RespHash, Response);
};
File “digtest.c”
#include <stdio.h>
#include "digcalc.h"
void main(int argc, char ** argv) {
char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
char * pszCNonce = "0a4f113b";
char * pszUser = "Mufasa";
char * pszRealm = "testrealm@host.com";
char * pszPass = "Circle Of Life";
char * pszAlg = "md5";
char szNonceCount[9] = "00000001";
char * pszMethod = "GET";
char * pszQop = "auth";
char * pszURI = "/dir/index.html";
HASHHEX HA1;
HASHHEX HA2 = "";
HASHHEX Response;
DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass,
pszNonce, pszCNonce, HA1);
DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
pszMethod, pszURI, HA2, Response);
printf("Response = %s\n", Response);
};
581
APPENDIX G
Language Tags
Language tags are short, standardized strings that name spoken languages—for
example, “fr” (French) and “en-GB” (Great Britain English). Each tag has one or
more parts, separated by hyphens, called subtags. Language tags were described in
detail in the section “Language Tags and HTTP” in Chapter 16.
This appendix summarizes the rules, standardized tags, and registration information
for language tags. It contains the following reference material:
• Rules for the first (primary) subtag are summarized in “First Subtag Rules.”
• Rules for the second subtag are summarized in “Second Subtag Rules.”
• IANA-registered language tags are shown in Table G-1.
• ISO 639 language codes are shown in Table G-2.
• ISO 3166 country codes are shown in Table G-3.
First Subtag Rules
If the first subtag is:
• Two characters long, it’s a language code from the ISO 639* and 639-1 standards
• Three characters long, it’s a language code listed in the ISO 639-2† standard
• The letter “i,” the language tag is explicitly IANA-registered
• The letter “x,” the language tag is a private, nonstandard, extension subtag
The ISO 639 and 639-2 names are summarized in Table G-2.
* See ISO standard 639, “Codes for the representation of names of languages.”
† See ISO 639-2, “Codes for the representation of names of languages—Part 2: Alpha-3 code.”
582 |Appendix G: Language Tags
Second Subtag Rules
If the second subtag is:
• Two characters long, it’s a country/region defined by ISO 3166*
• Three to eight characters long, it may be registered with the IANA
• One character long, it is illegal
The ISO 3166 country codes are summarized in Table G-3.
IANA-Registered Language Tags
* The country codes AA, QM–QZ, XA–XZ and ZZ are reserved by ISO 3166 as user-assigned codes. These
must not be used to form language tags.
Table G-1. Language tags
IANA language tag Description
i-bnn Bunun
i-default Default language context
i-hak Hakka
i-klingon Klingon
i-lux Luxembourgish
i-mingo Mingo
i-navajo Navajo
i-pwn Paiwan
i-tao Tao
i-tay Tayal
i-tsu Tsou
no-bok Norwegian “Book language”
no-nyn Norwegian “New Norwegian”
zh-gan Kan or Gan
zh-guoyu Mandarin or Standard Chinese
zh-hakka Hakka
zh-min Min, Fuzhou, Hokkien, Amoy, or Taiwanese
zh-wuu Shanghaiese or Wu
zh-xiang Xiang or Hunanese
zh-yue Cantonese
ISO 639 Language Codes |583
ISO 639 Language Codes
Table G-2. ISO 639 and 639-2 language codes
Language ISO 639 ISO 639-2
Abkhazian ab abk
Achinese ace
Acoli ach
Adangme ada
Afar aa aar
Afrihili afh
Afrikaans af afr
Afro-Asiatic (Other) afa
Akan aka
Akkadian akk
Albanian sq alb/sqi
Aleut ale
Algonquian languages alg
Altaic (Other) tut
Amharic am amh
Apache languages apa
Arabic ar ara
Aramaic arc
Arapaho arp
Araucanian arn
Arawak arw
Armenian hy arm/hye
Artificial (Other) art
Assamese as asm
Athapascan languages ath
Austronesian (Other) map
Avaric ava
Avestan ave
Awadhi awa
Aymara ay aym
Azerbaijani az aze
Aztec nah
Balinese ban
Baltic (Other) bat
584 |Appendix G: Language Tags
Baluchi bal
Bambara bam
Bamileke languages bai
Banda bad
Bantu (Other) bnt
Basa bas
Bashkir ba bak
Basque eu baq/eus
Beja bej
Bemba bem
Bengali bn ben
Berber (Other) ber
Bhojpuri bho
Bihari bh bih
Bikol bik
Bini bin
Bislama bi bis
Braj bra
Breton be bre
Buginese bug
Bulgarian bg bul
Buriat bua
Burmese my bur/mya
Byelorussian be bel
Caddo cad
Carib car
Catalan ca cat
Caucasian (Other) cau
Cebuano ceb
Celtic (Other) cel
Central American Indian (Other) cai
Chagatai chg
Chamorro cha
Chechen che
Cherokee chr
Cheyenne chy
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 639 Language Codes |585
Chibcha chb
Chinese zh chi/zho
Chinook jargon chn
Choctaw cho
Church Slavic chu
Chuvash chv
Coptic cop
Cornish cor
Corsican co cos
Cree cre
Creek mus
Creoles and Pidgins (Other) crp
Creoles and Pidgins, English-based (Other) cpe
Creoles and Pidgins, French-based (Other) cpf
Creoles and Pidgins, Portuguese-based (Other) cpp
Cushitic (Other) cus
Croatian hr
Czech cs ces/cze
Dakota dak
Danish da dan
Delaware del
Dinka din
Divehi div
Dogri doi
Dravidian (Other) dra
Duala dua
Dutch nl dut/nla
Dutch, Middle (ca. 1050-1350) dum
Dyula dyu
Dzongkha dz dzo
Efik efi
Egyptian (Ancient) egy
Ekajuk eka
Elamite elx
English en eng
English, Middle (ca. 1100-1500) enm
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
586 |Appendix G: Language Tags
English, Old (ca. 450-1100) ang
Eskimo (Other) esk
Esperanto eo epo
Estonian et est
Ewe ewe
Ewondo ewo
Fang fan
Fanti fat
Faroese fo fao
Fijian fj fij
Finnish fi fin
Finno-Ugrian (Other) fiu
Fon fon
French fr fra/fre
French, Middle (ca. 1400-1600) frm
French, Old (842- ca. 1400) fro
Frisian fy fry
Fulah ful
Ga gaa
Gaelic (Scots) gae/gdh
Gallegan gl glg
Ganda lug
Gayo gay
Geez gez
Georgian ka geo/kat
German de deu/ger
German, Middle High (ca. 1050-1500) gmh
German, Old High (ca. 750-1050) goh
Germanic (Other) gem
Gilbertese gil
Gondi gon
Gothic got
Grebo grb
Greek, Ancient (to 1453) grc
Greek, Modern (1453-) el ell/gre
Greenlandic kl kal
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 639 Language Codes |587
Guarani gn grn
Gujarati gu guj
Haida hai
Hausa ha hau
Hawaiian haw
Hebrew he heb
Herero her
Hiligaynon hil
Himachali him
Hindi hi hin
Hiri Motu hmo
Hungarian hu hun
Hupa hup
Iban iba
Icelandic is ice/isl
Igbo ibo
Ijo ijo
Iloko ilo
Indic (Other) inc
Indo-European (Other) ine
Indonesian id ind
Interlingua (IALA) ia ina
Interlingue ie ine
Inuktitut iu iku
Inupiak ik ipk
Iranian (Other) ira
Irish ga gai/iri
Irish, Old (to 900) sga
Irish, Middle (900 - 1200) mga
Iroquoian languages iro
Italian it ita
Japanese ja jpn
Javanese jv/jw jav/jaw
Judeo-Arabic jrb
Judeo-Persian jpr
Kabyle kab
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
588 |Appendix G: Language Tags
Kachin kac
Kamba kam
Kannada kn kan
Kanuri kau
Kara-Kalpak kaa
Karen kar
Kashmiri ks kas
Kawi kaw
Kazakh kk kaz
Khasi kha
Khmer km khm
Khoisan (Other) khi
Khotanese kho
Kikuyu kik
Kinyarwanda rw kin
Kirghiz ky kir
Komi kom
Kongo kon
Konkani kok
Korean ko kor
Kpelle kpe
Kru kro
Kuanyama kua
Kumyk kum
Kurdish ku kur
Kurukh kru
Kusaie kus
Kutenai kut
Ladino lad
Lahnda lah
Lamba lam
Langue d’Oc (post-1500) oc oci
Lao lo lao
Latin la lat
Latvian lv lav
Letzeburgesch ltz
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 639 Language Codes |589
Lezghian lez
Lingala ln lin
Lithuanian lt lit
Lozi loz
Luba-Katanga lub
Luiseno lui
Lunda lun
Luo (Kenya and Tanzania) luo
Macedonian mk mac/mak
Madurese mad
Magahi mag
Maithili mai
Makasar mak
Malagasy mg mlg
Malay ms may/msa
Malayalam mal
Maltese ml mlt
Mandingo man
Manipuri mni
Manobo languages mno
Manx max
Maori mi mao/mri
Marathi mr mar
Mari chm
Marshall mah
Marwari mwr
Masai mas
Mayan languages myn
Mende men
Micmac mic
Minangkabau min
Miscellaneous (Other) mis
Mohawk moh
Moldavian mo mol
Mon-Kmer (Other) mkh
Mongo lol
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
590 |Appendix G: Language Tags
Mongolian mn mon
Mossi mos
Multiple languages mul
Munda languages mun
Nauru na nau
Navajo nav
Ndebele, North nde
Ndebele, South nbl
Ndongo ndo
Nepali ne nep
Newari new
Niger-Kordofanian (Other) nic
Nilo-Saharan (Other) ssa
Niuean niu
Norse, Old non
North American Indian (Other) nai
Norwegian no nor
Norwegian (Nynorsk) nno
Nubian languages nub
Nyamwezi nym
Nyanja nya
Nyankole nyn
Nyoro nyo
Nzima nzi
Ojibwa oji
Oriya or ori
Oromo om orm
Osage osa
Ossetic oss
Otomian languages oto
Pahlavi pal
Palauan pau
Pali pli
Pampanga pam
Pangasinan pag
Panjabi pa pan
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 639 Language Codes |591
Papiamento pap
Papuan-Australian (Other) paa
Persian fa fas/per
Persian, Old (ca 600 - 400 B.C.) peo
Phoenician phn
Polish pl pol
Ponape pon
Portuguese pt por
Prakrit languages pra
Provencal, Old (to 1500) pro
Pushto ps pus
Quechua qu que
Rhaeto-Romance rm roh
Rajasthani raj
Rarotongan rar
Romance (Other) roa
Romanian ro ron/rum
Romany rom
Rundi rn run
Russian ru rus
Salishan languages sal
Samaritan Aramaic sam
Sami languages smi
Samoan sm smo
Sandawe sad
Sango sg sag
Sanskrit sa san
Sardinian srd
Scots sco
Selkup sel
Semitic (Other) sem
Serbian sr
Serbo-Croatian sh scr
Serer srr
Shan shn
Shona sn sna
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
592 |Appendix G: Language Tags
Sidamo sid
Siksika bla
Sindhi sd snd
Singhalese si sin
Sino-Tibetan (Other) sit
Siouan languages sio
Slavic (Other) sla
Siswant ss ssw
Slovak sk slk/slo
Slovenian sl slv
Sogdian sog
Somali so som
Songhai son
Sorbian languages wen
Sotho, Northern nso
Sotho, Southern st sot
South American Indian (Other) sai
Spanish es esl/spa
Sukuma suk
Sumerian sux
Sudanese su sun
Susu sus
Swahili sw swa
Swazi ssw
Swedish sv sve/swe
Syriac syr
Tagalog tl tgl
Tahitian tah
Tajik tg tgk
Tamashek tmh
Tamil ta tam
Tatar tt tat
Telugu te tel
Tereno ter
Thai th tha
Tibetan bo bod/tib
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 639 Language Codes |593
Tigre tig
Tigrinya ti tir
Timne tem
Tivi tiv
Tlingit tli
Tonga (Nyasa) to tog
Tonga (Tonga Islands) ton
Truk tru
Tsimshian tsi
Tsonga ts tso
Tswana tn tsn
Tumbuka tum
Turkish tr tur
Turkish, Ottoman (1500–1928) ota
Turkmen tk tuk
Tuvinian tyv
Twi tw twi
Ugaritic uga
Uighur ug uig
Ukrainian uk ukr
Umbundu umb
Undetermined und
Urdu ur urd
Uzbek uz uzb
Vai vai
Venda ven
Vietnamese vi vie
Volapük vo vol
Votic vot
Wakashan languages wak
Walamo wal
Waray war
Washo was
Welsh cy cym/wel
Wolof wo wol
Xhosa xh xho
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
594 |Appendix G: Language Tags
ISO 3166 Country Codes
Yakut sah
Yao yao
Yap yap
Yiddish yi yid
Yoruba yo yor
Zapotec zap
Zenaga zen
Zhuang za zha
Zulu zu zul
Zuni zun
Table G-3. ISO 3166 country codes
Country Code
Afghanistan AF
Albania AL
Algeria DZ
American Samoa AS
Andorra AD
Angola AO
Anguilla AI
Antarctica AQ
Antigua and Barbuda AG
Argentina AR
Armenia AM
Aruba AW
Australia AU
Austria AT
Azerbaijan AZ
Bahamas BS
Bahrain BH
Bangladesh BD
Barbados BB
Belarus BY
Belgium BE
Table G-2. ISO 639 and 639-2 language codes (continued)
Language ISO 639 ISO 639-2
ISO 3166 Country Codes |595
Belize BZ
Benin BJ
Bermuda BM
Bhutan BT
Bolivia BO
Bosnia and Herzegovina BA
Botswana BW
Bouvet Island BV
Brazil BR
British Indian Ocean Territory IO
Brunei Darussalam BN
Bulgaria BG
Burkina Faso BF
Burundi BI
Cambodia KH
Cameroon CM
Canada CA
Cape Verde CV
Cayman Islands KY
Central African Republic CF
Chad TD
Chile CL
China CN
Christmas Island CX
Cocos (Keeling) Islands CC
Colombia CO
Comoros KM
Congo CG
Congo (Democratic Republic of the) CD
Cook Islands CK
Costa Rica CR
Cote D’Ivoire CI
Croatia HR
Cuba CU
Cyprus CY
Czech Republic CZ
Table G-3. ISO 3166 country codes (continued)
Country Code
596 |Appendix G: Language Tags
Denmark DK
Djibouti DJ
Dominica DM
Dominican Republic DO
East Timor TP
Ecuador EC
Egypt EG
El Salvador SV
Equatorial Guinea GQ
Eritrea ER
Estonia EE
Ethiopia ET
Falkland Islands (Malvinas) FK
Faroe Islands FO
Fiji FJ
Finland FI
France FR
French Guiana GF
French Polynesia PF
French Southern Territories TF
Gabon GA
Gambia GM
Georgia GE
Germany DE
Ghana GH
Gibraltar GI
Greece GR
Greenland GL
Grenada GD
Guadeloupe GP
Guam GU
Guatemala GT
Guinea GN
Guinea-Bissau GW
Guyana GY
Haiti HT
Table G-3. ISO 3166 country codes (continued)
Country Code
ISO 3166 Country Codes |597
Heard Island and Mcdonald Islands HM
Holy See (Vatican City State) VA
Honduras HN
Hong Kong HK
Hungary HU
Iceland IS
India IN
Indonesia ID
Iran (Islamic Republic of) IR
Iraq IQ
Ireland IE
Israel IL
Italy IT
Jamaica JM
Japan JP
Jordan JO
Kazakstan KZ
Kenya KE
Kiribati KI
Korea (Democratic People’s Republic of) KP
Korea (Republic of) KR
Kuwait KW
Kyrgyzstan KG
Lao People’s Democratic Republic LA
Latvia LV
Lebanon LB
Lesotho LS
Liberia LR
Libyan Arab Jamahiriya LY
Liechtenstein LI
Lithuania LT
Luxembourg LU
Macau MO
Macedonia (The Former Yugoslav Republic of) MK
Madagascar MG
Malawi MW
Table G-3. ISO 3166 country codes (continued)
Country Code
598 |Appendix G: Language Tags
Malaysia MY
Maldives MV
Mali ML
Malta MT
Marshall Islands MH
Martinique MQ
Mauritania MR
Mauritius MU
Mayotte YT
Mexico MX
Micronesia (Federated States of) FM
Moldova (Republic of) MD
Monaco MC
Mongolia MN
Montserrat MS
Morocco MA
Mozambique MZ
Myanmar MM
Namibia NA
Nauru NR
Nepal NP
Netherlands NL
Netherlands Antilles AN
New Caledonia NC
New Zealand NZ
Nicaragua NI
Niger NE
Nigeria NG
Niue NU
Norfolk Island NF
Northern Mariana Islands MP
Norway NO
Oman OM
Pakistan PK
Palau PW
Palestinian Territory (Occupied) PS
Panama PA
Table G-3. ISO 3166 country codes (continued)
Country Code
ISO 3166 Country Codes |599
Papua New Guinea PG
Paraguay PY
Peru PE
Philippines PH
Pitcairn PN
Poland PL
Portugal PT
Puerto Rico PR
Qatar QA
Reunion RE
Romania RO
Russian Federation RU
Rwanda RW
Saint Helena SH
Saint Kitts and Nevis KN
Saint Lucia LC
Saint Pierre and Miquelon PM
Saint Vincent and the Grenadines VC
Samoa WS
San Marino SM
Sao Tome and Principe ST
Saudi Arabia SA
Senegal SN
Seychelles SC
Sierra Leone SL
Singapore SG
Slovakia SK
Slovenia SI
Solomon Islands SB
Somalia SO
South Africa ZA
South Georgia and the South Sandwich Islands GS
Spain ES
Sri Lanka LK
Sudan SD
Suriname SR
Svalbard and Jan Mayen SJ
Table G-3. ISO 3166 country codes (continued)
Country Code
600 |Appendix G: Language Tags
Swaziland SZ
Sweden SE
Switzerland CH
Syrian Arab Republic SY
Taiwan, Province of China TW
Tajikistan TJ
Tanzania (United Republic of) TZ
Thailand TH
Togo TG
Tokelau TK
Tonga TO
Trinidad and Tobago TT
Tunisia TN
Turkey TR
Turkmenistan TM
Turks and Caicos Islands TC
Tuvalu TV
Uganda UG
Ukraine UA
United Arab Emirates AE
United Kingdom GB
United States US
United States Minor Outlying Islands UM
Uruguay UY
Uzbekistan UZ
Vanuatu VU
Venezuela VE
Viet NAM VN
Virgin Islands (British) VG
Virgin ISLANDS (U.S.) VI
Wallis and Futuna WF
Western Sahara EH
Yemen YE
Yugoslavia YU
Zambia ZM
Table G-3. ISO 3166 country codes (continued)
Country Code
Language Administrative Organizations |601
Language Administrative Organizations
ISO 639 defines a maintenance agency for additions to and changes in the list of lan-
guages in ISO 639. This agency is:
International Information Centre for Terminology (Infoterm)
P.O. Box 130
A-1021 Wien
Austria
Phone: +43 1 26 75 35 Ext. 312
Fax: +43 1 216 32 72
ISO 639-2 defines a maintenance agency for additions to and changes in the list of
languages in ISO 639-2. This agency is:
Library of Congress
Network Development and MARC Standards Office
Washington, D.C. 20540
USA
Phone: +1 202 707 6237
Fax: +1 202 707 0115
URL: http://www.loc.gov/standards/iso639/
The maintenance agency for ISO 3166 (country codes) is:
ISO 3166 Maintenance Agency Secretariat
c/o DIN Deutsches Institut fuer Normung
Burggrafenstrasse 6
Postfach 1107
D-10787 Berlin
Germany
Phone: +49 30 26 01 320
Fax: +49 30 26 01 231
URL: http://www.din.de/gremien/nas/nabd/iso3166ma/
602
APPENDIX H
MIME Charset Registry
This appendix describes the MIME charset registry maintained by the Internet
Assigned Numbers Authority (IANA). A formatted table of charsets from the regis-
try is provided in Table H-1.
MIME Charset Registry
MIME charset tags are registered with the IANA (http://www.iana.org/numbers.htm).
The charset registry is a flat-file text database of records. Each record contains a char-
set name, reference citations, a unique MIB number, a source description, and a list
of aliases. A name or alias may be flagged “preferred MIME name.”
Here is the record for US-ASCII:
Name: ANSI_X3.4-1968 [RFC1345, KXS2]^
MIBenum: 3
Source: ECMA registry
Alias: iso-ir-6
Alias: ANSI_X3.4-1986
Alias: ISO_646.irv:1991
Alias: ASCII
Alias: ISO646-US
Alias: US-ASCII (preferred MIME name)
Alias: us
Alias: IBM367
Alias: cp367
Alias: csASCII
The procedure for registering a charset with the IANA is documented in RFC 2978
(http://www.ietf.org/rfc/rfc2978.txt).
Registered Charsets |603
Preferred MIME Names
Of the 235 charsets registered at the time of this writing, only 20 include “preferred
MIME names”—common charsets used by email and web applications. These are:
Registered Charsets
Table H-1 lists the contents of the charset registry as of March 2001. Refer directly to
http://www.iana.org for more information about the contents of this table.
Big5 EUC-JP EUC-KR
GB2312 ISO-2022-JP ISO-2022-JP-2
ISO-2022-KR ISO-8859-1 ISO-8859-2
ISO-8859-3 ISO-8859-4 ISO-8859-5
ISO-8859-6 ISO-8859-7 ISO-8859-8
ISO-8859-9 ISO-8859-10 KOI8-R
Shift-JIS US-ASCII
Table H-1. IANA MIME charset tags
Charset tag Aliases Description References
US-ASCII ANSI_X3.4-1968, iso-ir-6,
ANSI_X3.4-1986,
ISO_646.irv:1991, ASCII,
ISO646-US, us, IBM367,
cp367, csASCII
ECMA registry RFC1345, KXS2
ISO-10646-UTF-1 csISO10646UTF1 Universal Transfer Format (1)—this is
the multibyte encoding that subsets
ASCII-7; it does not have byte-ordering
issues
ISO_646.basic:1983 ref, csISO646basic1983 ECMA registry RFC1345, KXS2
INVARIANT csINVARIANT RFC1345, KXS2
ISO_646.irv:1983 iso-ir-2, irv,
csISO2IntlRefVersion
ECMA registry RFC1345, KXS2
BS_4730 iso-ir-4, ISO646-GB, gb, uk,
csISO4UnitedKingdom
ECMA registry RFC1345, KXS2
NATS-SEFI iso-ir-8-1, csNATSSEFI ECMA registry RFC1345, KXS2
NATS-SEFI-ADD iso-ir-8-2, csNATSSEFIADD ECMA registry RFC1345, KXS2
NATS-DANO iso-ir-9-1, csNATSDANO ECMA registry RFC1345, KXS2
NATS-DANO-ADD iso-ir-9-2, csNATSDANOADD ECMA registry RFC1345, KXS2
604 |Appendix H: MIME Charset Registry
SEN_850200_B iso-ir-10, FI, ISO646-FI,
ISO646-SE, se,
csISO10Swedish
ECMA registry RFC1345, KXS2
SEN_850200_C iso-ir-11, ISO646-SE2, se2,
csISO11SwedishForNames
ECMA registry RFC1345, KXS2
KS_C_5601-1987 iso-ir-149, KS_C_5601-1989,
KSC_5601, korean,
csKSC56011987
ECMA registry RFC1345, KXS2
ISO-2022-KR csISO2022KR RFC 1557 (see also KS_C_5601-1987) RFC1557, Choi
EUC-KR csEUCKR RFC 1557 (see also KS_C_5861-1992) RFC1557, Choi
ISO-2022-JP csISO2022JP RFC 1468 (see also RFC 2237) RFC1468,
Murai
ISO-2022-JP-2 csISO2022JP2 RFC 1554 RFC1554, Ohta
ISO-2022-CN RFC 1922 RFC1922
ISO-2022-CN-EXT RFC 1922 RFC1922
JIS_C6220-1969-jp JIS_C6220-1969, iso-ir-13,
katakana, x0201-7,
csISO13JISC6220jp
ECMA registry RFC1345, KXS2
JIS_C6220-1969-ro iso-ir-14, jp, ISO646-JP,
csISO14JISC6220ro
ECMA registry RFC1345, KXS2
IT iso-ir-15, ISO646-IT,
csISO15Italian
ECMA registry RFC1345, KXS2
PT iso-ir-16, ISO646-PT,
csISO16Portuguese
ECMA registry RFC1345, KXS2
ES iso-ir-17, ISO646-ES,
csISO17Spanish
ECMA registry RFC1345, KXS2
greek7-old iso-ir-18, csISO18Greek7Old ECMA registry RFC1345, KXS2
latin-greek iso-ir-19, csISO19LatinGreek ECMA registry RFC1345, KXS2
DIN_66003 iso-ir-21, de, ISO646-DE,
csISO21German
ECMA registry RFC1345, KXS2
NF_Z_62-010_(1973) iso-ir-25, ISO646-FR1,
csISO25French
ECMA registry RFC1345, KXS2
Latin-greek-1 iso-ir-27, csISO27LatinGreek1 ECMA registry RFC1345, KXS2
ISO_5427 iso-ir-37, csISO5427Cyrillic ECMA registry RFC1345, KXS2
JIS_C6226-1978 iso-ir-42,
csISO42JISC62261978
ECMA registry RFC1345, KXS2
BS_viewdata iso-ir-47, csISO47BSViewdata ECMA registry RFC1345, KXS2
INIS iso-ir-49, csISO49INIS ECMA registry RFC1345, KXS2
INIS-8 iso-ir-50, csISO50INIS8 ECMA registry RFC1345, KXS2
INIS-cyrillic iso-ir-51, csISO51INISCyrillic ECMA registry RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |605
ISO_5427:1981 iso-ir-54, ISO5427Cyrillic1981 ECMA registry RFC1345, KXS2
ISO_5428:1980 iso-ir-55, csISO5428Greek ECMA registry RFC1345, KXS2
GB_1988-80 iso-ir-57, cn, ISO646-CN,
csISO57GB1988
ECMA registry RFC1345, K5,
KXS2
GB_2312-80 iso-ir-58, chinese,
csISO58GB231280
ECMA registry RFC1345, KXS2
NS_4551-1 iso-ir-60, ISO646-NO, no,
csISO60DanishNorwegian,
csISO60Norwegian1
ECMA registry RFC1345, KXS2
NS_4551-2 ISO646-NO2, iso-ir-61, no2,
csISO61Norwegian2
ECMA registry RFC1345, KXS2
NF_Z_62-010 iso-ir-69, ISO646-FR, fr,
csISO69French
ECMA registry RFC1345, KXS2
videotex-suppl iso-ir-70,
csISO70VideotexSupp1
ECMA registry RFC1345, KXS2
PT2 iso-ir-84, ISO646-PT2,
csISO84Portuguese2
ECMA registry RFC1345, KXS2
ES2 iso-ir-85, ISO646-ES2,
csISO85Spanish2
ECMA registry RFC1345, KXS2
MSZ_7795.3 iso-ir-86, ISO646-HU, hu,
csISO86Hungarian
ECMA registry RFC1345, KXS2
JIS_C6226-1983 iso-ir-87, x0208,
JIS_X0208-1983,
csISO87JISX0208
ECMA registry RFC1345, KXS2
greek7 iso-ir-88, csISO88Greek7 ECMA registry RFC1345, KXS2
ASMO_449 ISO_9036, arabic7, iso-ir-89,
csISO89ASMO449
ECMA registry RFC1345, KXS2
iso-ir-90 csISO90 ECMA registry RFC1345, KXS2
JIS_C6229-1984-a iso-ir-91, jp-ocr-a,
csISO91JISC62291984a
ECMA registry RFC1345, KXS2
JIS_C6229-1984-b iso-ir-92, ISO646-JP-OCR-B,
jp-ocr-b,
csISO92JISC62991984b
ECMA registry RFC1345, KXS2
JIS_C6229-1984-b-add iso-ir-93, jp-ocr-b-add,
csISO93JIS62291984badd
ECMA registry RFC1345, KXS2
JIS_C6229-1984-hand iso-ir-94, jp-ocr-hand,
csISO94JIS62291984hand
ECMA registry RFC1345, KXS2
JIS_C6229-1984-hand-add iso-ir-95, jp-ocr-hand-add,
csISO95JIS62291984handadd
ECMA registry RFC1345, KXS2
JIS_C6229-1984-kana iso-ir-96,
csISO96JISC62291984kana
ECMA registry RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
606 |Appendix H: MIME Charset Registry
ISO_2033-1983 iso-ir-98, e13b, csISO2033 ECMA registry RFC1345, KXS2
ANSI_X3.110-1983 iso-ir-99, CSA_T500-1983,
NAPLPS, csISO99NAPLPS
ECMA registry RFC1345, KXS2
ISO-8859-1 ISO_8859-1:1987, iso-ir-100,
ISO_8859-1, latin1, l1,
IBM819, CP819, csISOLatin1
ECMA registry RFC1345, KXS2
ISO-8859-2 ISO_8859-2:1987, iso-ir-101,
ISO_8859-2, latin2, l2,
csISOLatin2
ECMA registry RFC1345, KXS2
T.61-7bit iso-ir-102, csISO102T617bit ECMA registry RFC1345, KXS2
T.61-8bit T.61, iso-ir-103,
csISO103T618bit
ECMA registry RFC1345, KXS2
ISO-8859-3 ISO_8859-3:1988, iso-ir-109,
ISO_8859-3, latin3, l3,
csISOLatin3
ECMA registry RFC1345, KXS2
ISO-8859-4 ISO_8859-4:1988, iso-ir-110,
ISO_8859-4, latin4, l4,
csISOLatin4
ECMA registry RFC1345, KXS2
ECMA-cyrillic iso-ir-111,
csISO111ECMACyrillic
ECMA registry RFC1345, KXS2
CSA_Z243.4-1985-1 iso-ir-121, ISO646-CA, csa7-1,
ca, csISO121Canadian1
ECMA registry RFC1345, KXS2
CSA_Z243.4-1985-2 iso-ir-122, ISO646-CA2,
csa7-2, csISO122Canadian2
ECMA registry RFC1345, KXS2
CSA_Z243.4-1985-gr iso-ir-123,
csISO123CSAZ24341985gr
ECMA registry RFC1345, KXS2
ISO-8859-6 ISO_8859-6:1987, iso-ir-127,
ISO_8859-6, ECMA-114,
ASMO-708, arabic,
csISOLatinArabic
ECMA registry RFC1345, KXS2
ISO_8859-6-E csISO88596E RFC 1556 RFC1556, IANA
ISO_8859-6-I csISO88596I RFC 1556 RFC1556, IANA
ISO-8859-7 ISO_8859-7:1987, iso-ir-126,
ISO_8859-7, ELOT_928,
ECMA-118, greek, greek8,
csISOLatinGreek
ECMA registry RFC1947,
RFC1345, KXS2
T.101-G2 iso-ir-128, csISO128T101G2 ECMA registry RFC1345, KXS2
ISO-8859-8 ISO_8859-8:1988, iso-ir-138,
ISO_8859-8, hebrew,
csISOLatinHebrew
ECMA registry RFC1345, KXS2
ISO_8859-8-E csISO88598E RFC 1556 RFC1556,
Nussbacher
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |607
ISO_8859-8-I csISO88598I RFC 1556 RFC1556,
Nussbacher
CSN_369103 iso-ir-139,
csISO139CSN369103
ECMA registry RFC1345, KXS2
JUS_I.B1.002 iso-ir-141, ISO646-YU, js, yu,
csISO141JUSIB1002
ECMA registry RFC1345, KXS2
ISO_6937-2-add iso-ir-142, csISOTextComm ECMA registry and ISO 6937-2:1983 RFC1345, KXS2
IEC_P27-1 iso-ir-143, csISO143IECP271 ECMA registry RFC1345, KXS2
ISO-8859-5 ISO_8859-5:1988, iso-ir-144,
ISO_8859-5, cyrillic,
csISOLatinCyrillic
ECMA registry RFC1345, KXS2
JUS_I.B1.003-serb iso-ir-146, serbian,
csISO146Serbian
ECMA registry RFC1345, KXS2
JUS_I.B1.003-mac macedonian, iso-ir-147,
csISO147Macedonian
ECMA registry RFC1345, KXS2
ISO-8859-9 ISO_8859-9:1989, iso-ir-148,
ISO_8859-9, latin5, l5,
csISOLatin5
ECMA registry RFC1345, KXS2
greek-ccitt iso-ir-150, csISO150,
csISO150GreekCCITT
ECMA registry RFC1345, KXS2
NC_NC00-10:81 cuba, iso-ir-151, ISO646-CU,
csISO151Cuba
ECMA registry RFC1345, KXS2
ISO_6937-2-25 iso-ir-152, csISO6937Add ECMA registry RFC1345, KXS2
GOST_19768-74 ST_SEV_358-88, iso-ir-153,
csISO153GOST1976874
ECMA registry RFC1345, KXS2
ISO_8859-supp iso-ir-154, latin1-2-5,
csISO8859Supp
ECMA registry RFC1345, KXS2
ISO_10367-box iso-ir-155, csISO10367Box ECMA registry RFC1345, KXS2
ISO-8859-10 iso-ir-157, l6,
ISO_8859-10:1992,
csISOLatin6, latin6
ECMA registry RFC1345, KXS2
latin-lap lap, iso-ir-158, csISO158Lap ECMA registry RFC1345, KXS2
JIS_X0212-1990 x0212, iso-ir-159,
csISO159JISX02121990
ECMA registry RFC1345, KXS2
DS_2089 DS2089, ISO646-DK, dk,
csISO646Danish
Danish Standard, DS 2089, February
1974
RFC1345, KXS2
us-dk csUSDK RFC1345, KXS2
dk-us csDKUS RFC1345, KXS2
JIS_X0201 X0201, csHalfWidthKatakana JIS X 0201-1976—1 byte only; this is
equivalent to JIS/Roman (similar to
ASCII) plus 8-bit half-width katakana
RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
608 |Appendix H: MIME Charset Registry
KSC5636 ISO646-KR, csKSC5636 RFC1345, KXS2
ISO-10646-UCS-2 csUnicode The 2-octet Basic Multilingual Plane,
a.k.a. Unicode—this needs to specify
network byte order; the standard does
not specify it (it is a 16-bit integer
space)
ISO-10646-UCS-4 csUCS4 The full code space (same comment
about byte order; these are 31-bit
numbers)
DEC-MCS dec, csDECMCS VAX/VMS User’s Manual, Order
Number: AI-Y517A-TE, April 1986
RFC1345, KXS2
hp-roman8 roman8, r8, csHPRoman8 LaserJet IIP Printer User’s Manual, HP
part no 33471-90901, Hewlett-
Packard, June 1989
HP-PCL5,
RFC1345, KXS2
macintosh mac, csMacintosh The Unicode Standard v1.0, ISBN
0201567881, Oct 1991
RFC1345, KXS2
IBM037 cp037, ebcdic-cp-us,
ebcdic-cp-ca, ebcdic-cp-wt,
ebcdic-cp-nl, csIBM037
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM038 EBCDIC-INT, cp038, csIBM038 IBM 3174 Character Set Ref, GA27-
3831-02, March 1990
RFC1345, KXS2
IBM273 CP273, csIBM273 IBMNLS RMVol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM274 EBCDIC-BE, CP274, csIBM274 IBM 3174 Character Set Ref, GA27-
3831-02, March 1990
RFC1345, KXS2
IBM275 EBCDIC-BR, cp275, csIBM275 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM277 EBCDIC-CP-DK,EBCDIC-CP-NO,
csIBM277
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM278 CP278, ebcdic-cp-fi,
ebcdic-cp-se, csIBM278
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM280 CP280, ebcdic-cp-it, csIBM280 IBM NLS RM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM281 EBCDIC-JP-E, cp281, csIBM281 IBM 3174 Character Set Ref, GA27-
3831-02, March 1990
RFC1345, KXS2
IBM284 CP284, ebcdic-cp-es,
csIBM284
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM285 CP285, ebcdic-cp-gb,
csIBM285
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM290 cp290, EBCDIC-JP-kana,
csIBM290
IBM 3174 Character Set Ref, GA27-
3831-02, March 1990
RFC1345, KXS2
IBM297 cp297, ebcdic-cp-fr, csIBM297 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |609
IBM420 cp420, ebcdic-cp-ar1,
csIBM420
IBM NLS RM Vol2 SE09-8002-01, March
1990, IBM NLS RM p 11-11
RFC1345, KXS2
IBM423 cp423, ebcdic-cp-gr,
csIBM423
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM424 cp424, ebcdic-cp-he,
csIBM424
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM437 cp437, 437,
csPC8CodePage437
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM500 CP500, ebcdic-cp-be,
ebcdic-cp-ch, csIBM500
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM775 cp775, csPC775Baltic HP PCL 5 Comparison Guide (P/N 5021-
0329) pp B-13, 1996
HP-PCL5
IBM850 cp850, 850,
csPC850Multilingual
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM851 cp851, 851, csIBM851 IBM NLSRM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM852 cp852, 852, csPCp852 IBM NLS RM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM855 cp855, 855, csIBM855 IBM NLSRM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM857 cp857, 857, csIBM857 IBM NLSRM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM860 cp860, 860, csIBM860 IBM NLSRM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM861 cp861, 861, cp-is, csIBM861 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM862 cp862, 862,
csPC862LatinHebrew
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM863 cp863, 863, csIBM863 IBM keyboard layouts and code pages,
PN 07G4586, June 1991
RFC1345, KXS2
IBM864 cp864, csIBM864 IBM keyboard layouts and code pages,
PN 07G4586, June 1991
RFC1345, KXS2
IBM865 cp865, 865, csIBM865 IBM DOS 3.3 Ref (Abridged), 94X9575,
Feb 1987
RFC1345, KXS2
IBM866 cp866, 866, csIBM866 IBM NLDG Vol2 SE09-8002-03, August
1994
Pond
IBM868 CP868, cp-ar, csIBM868 IBM NLSRM Vol2 SE09-8002-01,March
1990
RFC1345, KXS2
IBM869 cp869, 869, cp-gr, csIBM869 IBM keyboard layouts and code pages,
PN 07G4586, June 1991
RFC1345, KXS2
IBM870 CP870, ebcdic-cp-roece,
ebcdic-cp-yu, csIBM870
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
610 |Appendix H: MIME Charset Registry
IBM871 CP871, ebcdic-cp-is, csIBM871 IBMNLS RM Vol2 SE09-8002-01,March
1990
RFC1345, KXS2
IBM880 cp880, EBCDIC-Cyrillic,
csIBM880
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM891 cp891, csIBM891 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM903 cp903, csIBM903 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM904 cp904, 904, csIBBM904 IBM NLSRM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
IBM905 CP905, ebcdic-cp-tr, csIBM905 IBM 3174 Character Set Ref, GA27-
3831-02, March 1990
RFC1345, KXS2
IBM918 CP918, ebcdic-cp-ar2,
csIBM918
IBM NLS RM Vol2 SE09-8002-01, March
1990
RFC1345, KXS2
IBM1026 CP1026, csIBM1026 IBM NLS RM Vol2SE09-8002-01, March
1990
RFC1345, KXS2
EBCDIC-AT-DE csIBMEBCDICATDE IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-AT-DE-A csEBCDICATDEA IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-CA-FR csEBCDICCAFR IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-DK-NO csEBCDICDKNO IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-DK-NO-A csEBCDICDKNOA IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-FI-SE csEBCDICFISE IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-FI-SE-A csEBCDICFISEA IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-FR csEBCDICFR IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-IT csEBCDICIT IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-PT IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-ES csEBCDICES IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-ES-A csEBCDICESA IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |611
EBCDIC-ES-S csEBCDICESS IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-UK csEBCDICUK IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
EBCDIC-US csEBCDICUS IBM 3270 Char Set Ref Ch 10, GA27-
2837-9, April 1987
RFC1345, KXS2
UNKNOWN-8BIT csUnknown8BiT RFC1428
MNEMONIC csMnemonic RFC 1345, also known as
“mnemonic+ascii+38”
RFC1345, KXS2
MNEM csMnem RFC 1345, also known as
“mnemonic+ascii+8200”
RFC1345, KXS2
VISCII csVISCII RFC 1456 RFC1456
VIQR csVIQR RFC 1456 RFC1456
KOI8-R csKOI8R RFC 1489, based on GOST-19768-74,
ISO-6937/8, INIS-Cyrillic, ISO-5427
RFC1489
KOI8-U RFC 2319 RFC2319
IBM00858 CCSID00858, CP00858, PC-
Multilingual-850+euro
IBM (see .../assignments/character-
set-info/IBM00858) [Mahdi]
IBM00924 CCSID00924, CP00924, ebcdic-
Latin9--euro
IBM (see .../assignments/character-
set-info/IBM00924) [Mahdi]
IBM01140 CCSID01140, CP01140, ebcdic-
us-37+euro
IBM (see .../assignments/character-
set-info/IBM01140) [Mahdi]
IBM01141 CCSID01141, CP01141, ebcdic-
de-273+euro
IBM (see .../assignments/character-
set-info/IBM01141) [Mahdi]
IBM01142 CCSID01142, CP01142, ebcdic-
dk-277+euro, ebcdic-no-
277+euro
IBM (see .../assignments/character-
set-info/IBM01142) [Mahdi]
IBM01143 CCSID01143, CP01143, ebcdic-
fi-278+euro, ebcdic-se-
278+euro
IBM (see .../assignments/character-
set-info/IBM01143) [Mahdi]
IBM01144 CCSID01144, CP01144, ebcdic-
it-280+euro
IBM (see .../assignments/character-
set-info/IBM01144) [Mahdi]
IBM01145 CCSID01145, CP01145, ebcdic-
es-284+euro
IBM (see .../assignments/character-
set-info/IBM01145) [Mahdi]
IBM01146 CCSID01146, CP01146, ebcdic-
gb-285+euro
IBM (see .../assignments/character-
set-info/IBM01146) [Mahdi]
IBM01147 CCSID01147, CP01147, ebcdic-
fr-297+euro
IBM (see .../assignments/character-
set-info/IBM01147) [Mahdi]
IBM01148 CCSID01148, CP01148, ebcdic-
international-500+euro
IBM (see .../assignments/character-
set-info/IBM01148) [Mahdi]
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
612 |Appendix H: MIME Charset Registry
IBM01149 CCSID01149, CP01149, ebcdic-
is-871+euro
IBM (see .../assignments/character-
set-info/IBM01149) [Mahdi]
Big5-HKSCS None See (.../assignments/character-set-
info/Big5-HKSCS) [Yick]
UNICODE-1-1 csUnicode11 RFC 1641 RFC1641
SCSU None SCSU (see .../assignments/character-
set-info/SCSU) [Scherer]
UTF-7 None RFC 2152 RFC2152
UTF-16BE None RFC 2781 RFC2781
UTF-16LE None RFC 2781 RFC2781
UTF-16 None RFC 2781 RFC2781
UNICODE-1-1-UTF-7 csUnicode11UTF7 RFC 1642 RFC1642
UTF-8 RFC 2279 RFC2279
iso-8859-13 ISO (see ...assignments/character-set-
info/iso-8859-13)[Tumasonis]
iso-8859-14 iso-ir-199,
ISO_8859-14:1998,
ISO_8859-14, latin8,
iso-celtic, l8
ISO (see ...assignments/character-set-
info/iso-8859-14) [Simonsen]
ISO-8859-15 ISO_8859-15 ISO
JIS_Encoding csJISEncoding JIS X 0202-1991; uses ISO 2022 escape
sequences to shift code sets, as
documented in JIS X 0202-1991
Shift_JIS MS_Kanji, csShiftJIS This charset is an extension of
csHalfWidthKatakana—it adds
graphic characters in JIS X 0208. The
CCSs areJIS X0201:1997 and JIS X0208:
1997. The complete definition is shown
in Appendix 1 of JISX0208:1997. This
charset can be used for the top-level
media type “text”.
EUC-JP Extended_UNIX_Code_
Packed_Format_for_
Japanese,
csEUCPkdFmtJapanese
Standardized by OSF, UNIX
International, and UNIX Systems
Laboratories Pacific. Uses ISO 2022
rules to select code set. code set 0: US-
ASCII (a single 7-bit byte set); code set
1: JIS X0208-1990 (a double 8-bit byte
set) restricted to A0–FF in both bytes;
code set 2: half-width katakana (a
single 7-bit byte set) requiring SS2 as
the character prefix; code set 3: JIS
X0212-1990 (a double 7-bit byte set)
restricted to A0–FF in both bytes
requiring SS3 as the character prefix.
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |613
Extended_UNIX_Code_
Fixed_Width_for_
Japanese
csEUCFixWidJapanese Used in Japan. Each character is 2
octets. code set 0: US-ASCII (a single 7-
bit byte set), 1st byte = 00, 2nd byte =
20–7E; code set 1: JIS X0208-1990 (a
double 7-bit byte set) restricted to A0–
FF in both bytes; code set 2: half-width
katakana (a single 7-bit byte set), 1st
byte = 00, 2nd byte = A0–FF; code set
3: JIS X0212-1990 (a double 7-bit byte
set) restricted to A0–FF in the first byte
and 21–7E in the second byte.
ISO-10646-UCS-Basic csUnicodeASCII ASCII subset of Unicode. Basic Latin =
collection 1. See ISO 10646, Appendix A.
ISO-10646-Unicode-Latin1 csUnicodeLatin1, ISO-10646 ISO Latin-1 subset of Unicode. Basic
Latin and Latin-1. Supplement =
collections 1 and 2. See ISO 10646,
Appendix A, and RFC 1815.
ISO-10646-J-1 ISO 10646 Japanese. See RFC 1815.
ISO-Unicode-IBM-1261 csUnicodeIBM1261 IBM Latin-2, -3, -5, Extended
Presentation Set, GCSGID: 1261
ISO-Unicode-IBM-1268 csUnidoceIBM1268 IBM Latin-4 Extended Presentation Set,
GCSGID: 1268
ISO-Unicode-IBM-1276 csUnicodeIBM1276 IBM Cyrillic Greek Extended
Presentation Set, GCSGID: 1276
ISO-Unicode-IBM-1264 csUnicodeIBM1264 IBM Arabic Presentation Set, GCSGID:
1264
ISO-Unicode-IBM-1265 csUnicodeIBM1265 IBM Hebrew Presentation Set, GCSGID:
1265
ISO-8859-1-Windows-3.0-
Latin-1
csWindows30Latin1 Extended ISO 8859-1 Latin-1 for
Windows 3.0. PCL Symbol Set ID: 9U.
HP-PCL5
ISO-8859-1-Windows-3.1-
Latin-1
csWindows31Latin1 Extended ISO 8859-1 Latin-1 for
Windows 3.1. PCL Symbol Set ID: 19U.
HP-PCL5
ISO-8859-2-Windows-
Latin-2
csWindows31Latin2 Extended ISO 8859-2. Latin-2 for
Windows 3.1. PCL Symbol Set ID: 9E.
HP-PCL5
ISO-8859-9-Windows-
Latin-5
csWindows31Latin5 Extended ISO 8859-9. Latin-5 for
Windows 3.1. PCL Symbol Set ID: 5T.
HP-PCL5
Adobe-Standard-Encoding csAdobeStandardEncoding PostScript Language Reference
Manual. PCL Symbol Set ID: 10J.
Adobe
Ventura-US csVenturaUS Ventura US-ASCII plus characters
typically used in publishing, such as
pilcrow, copyright, registered,
trademark, section, dagger, and
double dagger in the range A0 (hex) to
FF (hex). PCL Symbol Set ID: 14J.
HP-PCL5
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
614 |Appendix H: MIME Charset Registry
Ventura-International csVenturaInternational Ventura International. ASCII plus coded
characters similar to Roman8. PCL
Symbol Set ID: 13J.
HP-PCL5
PC8-Danish-Norwegian csPC8DanishNorwegian PC Danish Norwegian 8-bit PC set for
Danish Norwegian. PCL Symbol Set ID:
11U.
HP-PCL5
PC8-Turkish csPC8Turkish PC Latin Turkish. PCL Symbol Set ID: 9T. HP-PCL5
IBM-Symbols csIBMSymbols Presentation Set, CPGID: 259 IBM-CIDT
IBM-Thai csIBMThai Presentation Set, CPGID: 838 IBM-CIDT
HP-Legal csHPLegal PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 1U.
HP-PCL5
HP-Pi-font csHPPiFont PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 15U.
HP-PCL5
HP-Math8 csHPMath8 PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 8M.
HP-PCL5
Adobe-Symbol-Encoding csHPPSMath PostScript Language Reference
Manual. PCL Symbol Set ID: 5M.
Adobe
HP-DeskTop csHPDesktop PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 7J.
HP-PCL5
Ventura-Math csVenturaMath PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 6M.
HP-PCL5
Microsoft-Publishing csMicrosoftPublishing PCL 5 Comparison Guide, Hewlett-
Packard, HP part number 5961-0510,
October 1992. PCL Symbol Set ID: 6J.
HP-PCL5
Windows-31J csWindows31J Windows Japanese. A further
extension of Shift_JIS to include NEC
special characters (Row 13), NEC
selectionofIBMextensions(Rows89to
92), and IBM extensions (Rows 115 to
119). The CCSs are JIS X0201:1997, JIS
X0208:1997,and these extensions. This
charset can be used for the top-level
media type “text”, but it is of limited or
specialized use (see RFC 2278). PCL
Symbol Set ID: 19K.
GB2312 csGB2312 Chinese for People’s Republic of China
(PRC) mixed 1-byte, 2-byte set: 20–7E
= 1-byte ASCII; A1–FE = 2-byte PRC
Kanji. See GB 2312-80. PCL Symbol Set
ID: 18C.
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
Registered Charsets |615
Big5 csBig5 Chinese for Taiwan Multibyte set. PCL
Symbol Set id: 18T.
windows-1250 Microsoft (see .../character-set-info/
windows-1250) [Lazhintseva]
windows-1251 Microsoft (see .../character-set-info/
windows-1251) [Lazhintseva]
windows-1252 Microsoft (see .../character-set-info/
windows-1252) [Wendt]
windows-1253 Microsoft (see .../character-set-info/
windows-1253) [Lazhintseva]
windows-1254 Microsoft (see .../character-set-info/
windows-1254) [Lazhintseva]
windows-1255 Microsoft (see .../character-set-info/
windows-1255) [Lazhintseva]
windows-1256 Microsoft (see .../character-set-info/
windows-1256) [Lazhintseva]
windows-1257 Microsoft (see .../character-set-info/
windows-1257) [Lazhintseva]
windows-1258 Microsoft (see .../character-set-info/
windows-1258) [Lazhintseva]
TIS-620 Thai Industrial Standards Institute
(TISI)
[Tantsetthi]
HZ-GB-2312 RFC 1842, RFC 1843 [RFC1842,
RFC1843]
Table H-1. IANA MIME charset tags (continued)
Charset tag Aliases Description References
617
We’d like to hear your suggestions for improving our indexes. Send email to index@oreilly.com.
Index
Symbols
: (colon), use in headers, 47
= (equals sign), base-64 encoding, 572
/~ (slash-tilde), 122
Numbers
8-bit identity encoding, 382
100 Continue status code, 59, 60
100-199 status codes, 59–60, 505
200-299 status codes, 61, 505
300-399 status codes, 61–64, 506
400-499 status codes, 65–66, 506
500-599 status codes, 66, 507
2MSL (maximum segment lifetime), 85
A
absolute URLs, 30
Accept headers, 69, 508
robots and, 225
Accept-Charset headers, 371, 375, 509
MIME charset encoding tags and, 374
Accept-Encoding headers, 509
Accept-Instance-Manipulation headers, 367
Accept-Language headers, 371, 385, 510
content negotiation and, 398
Accept-Ranges headers, 510
access controls, 124
proxy authentication, 156
access proxies, 137
advertising, hit counts and caches, 194–196
age and freshness lifetime, 188
Age headers, 510
agents, 19
algorithms
aging and freshness, 187–194
document age calculation, 189–194
instance-manipulation algorithms, 367
LM-Factor, 184
message digest algorithms, 291–294
symmetric authentication, 298
Nagle’s algorithm, 84
redirection, enhanced DNS-based, 457
resource-discovery algorithm
(WPAD), 143, 465
RSA, 317
aliases (URLs), 219
Allow headers, 159, 511
<allprop> element, 437
anonymizers, 136
anycast addressing, 457
Apache web servers, 110
content negotiation, 399
MultiViews directive, 400
type-map files, 399
DirectoryIndex configuration
directive, 123
document root, setting, 121
HostnameLookups configuration
directive, 115
HTTP headers, control of, 186
IdentityCheck configuration
directive, 116
magic typing, 126
APIs (application programming
interfaces), 203
server extensions, 205
web services and, 205
618 | Index
application/* MIME types, 540–557
application programming interfaces (see
APIs)
application servers, 123, 203
ASCII character set, 379
asymmetric cryptography, 315
attacks, 303–306
batched brute-force attacks, 305
chosen plaintext attacks, 305
dictionary attacks, 304
enumeration, 313
evidence of, 301
header tampering, 303
hostile proxies, 304
man-in-the-middle attacks, 304
replay attacks, 284, 303
preventing, 289
audio/* MIME types, 557–559
authentication, 277–280
basic (see basic authentication)
challenge/response framework, 278
digest (see digest authentication)
headers, 278
HTCP, 480
multiple authentication schemes, risks
of, 303
protocols, 278
proxy servers, 156
server, using digital certificates, 321
(see also HTTPS)
Authentication-Info directives, 576
Authorization headers, 281, 511
directives, 575
preemptive generation, 295
automatic expansion of URLs, 30
B
bandwidth
bottlenecks, 161
transfer times and, 162
base URLs, 32
base-64 encoding, 570–572
alphabet, 571
equals sign (=), 572
HTTP, compatibility with, 570
padding, 572
Perl implementation, 572
purpose, 570
username/password, 282
bases, 31
basic authentication, 281–284
example, 281
headers, 281, 300
insecurity of, 283, 286
protection space, 302
by proxy servers, 283
username/password encoding, 282
web server vs. proxy, 283
(see also authentication)
batched brute-force attacks, 305
Binary Wire Protocol, 250, 252
blind relays, 94
browsers
Host headers and older versions of, 419
HTTP, use of, 13
parallel connections, maximum, 90
URLs, automatic expansion of, 34
byte hit rate, 167
C
Cache Array Routing Protocol (see CARP)
Cache-Control headers, 175–177, 182–186,
189, 511
directives, 361
caches, 18, 133, 161–196
advertising and, 194–196
aging and freshness algorithms, 187–194
byte hit rate, 167
cache busting, 492
cache hits and misses, 165, 168
cache meshes, 170
cache validators, 181
Cache-Control headers, 175–177,
182–186, 189, 511
directives, 361
cookies and, 273
digest authentication and, 302
distance delays and, 163
DNS caching and load balancing, 456
document expiration, 175
exclusion of documents from, 182
expiration time, setting, 182
Expires headers, 175, 183
flash crowds and, 163
freshness check, 173
heuristic expiration, 184
hit logs, 195
hit rate, 167
HTTP-EQUIV tag, 187
Index | 619
logging, 174
lookup, 173
maintaining currency, 175
message truncation and, 344
network bottlenecks and, 161
parent caches, 169
parsing, 172
peering, 171
processing steps, 171–175
flowchart, 175
receiving, 172
response creation, 174
revalidate hits, 165
revalidations, 165–166
sending, 174
server revalidation, 175, 177
setting controls, 186
sibling caches, 171
surrogate caches, 421
topologies, 168
uses, 161
caching headers, 68
caching proxies (see caches)
caching proxy servers, 169
canonicalizing of URLs, 37, 220
CARP (Cache Array Routing
Protocol), 475–478
disadvantages, 476
ICP vs., 475
redirection method, 476
CAs (certificate authorities), 327
case sensitivity, language tags, 386
CDNs (content distribution networks), 421
certificate authorities (CAs), 327
certificates (see digital certificates)
CGI (Common Gateway Interface), 203, 204
challenge/response authentication
model, 278
challenge headers, 72
multiple challenges, 301
character encoding schemes, 377, 381
character repertoire, 377
character sets, 35, 371–376
encoding, 370
mechanisms, 36
restricted characters, 36
characters, 378
URLs, legal in, 35
charset tags, 371
IANA MIME character set registry
and, 371
chemical/* MIME types, 559–561
child filters, 131
chosen plaintext attacks, 305
chunked encodings, 345
ciphers (see under cryptography)
ciphertext, 310
cleartext, 310
client error status codes (400-499), 65–66,
505
client hostname identification, 115
client proxy configuration, 141–144
manual, 142
PAC (Proxy Auto-configuration)
protocol, 142, 463
WPAD (Web Proxy Autodiscovery
Protocol), 143
client-driven negotiation, 396
disadvantages, 396
Client-ip headers, 258, 260, 512
clients
100 Continue status code and, 59
freshness constraints, 185
Host header requirements, 418
identification, 257–276
fat URLs, using for, 262
IP addresses, using for, 259
user logins, using for, 260
(see also cookies)
supported character sets, 375
client-side gateways, 199
security accelerator gateways, 202
client-side state, 265
code width, 377
coded character sets, 377, 379
coded characters, 376
coding space, 376
collections, 439
collisions, one-way digests, 288
colon (:), use in headers, 47
Combined Log Format, 485
Common Gateway Interface (CGI), 203, 204
Common Log Format, 484
composite MIME types, 534
conditional requests, 362
headers, 70, 178–181
configuration URLs (CURLs), 465
CONNECT method, 206–208, 336
connection handshake delays, 82
Connection headers, 86, 512
connections (see HTTP connections)
content distribution networks (CDNs), 421
620 | Index
content encodings, 345, 351–354
content injection, 405
content negotiation, 395–403
on Apache web servers, 399
client-driven negotiation, 396
headers, 397
other protocols and, 405
performance limitations, 405
quality values, 398
server-driven negotiation, 397–400
techniques, 395
transparent negotiation, 400–403
content routers, 134, 170
Content-Base headers, 513
Content-Encoding headers, 513
content-injection, 405
Content-Language headers, 371, 384, 513
Content-Length headers, 344–347, 514
content encoding and, 345
persistent connections and, 345
Content-Location headers, 514
Content-MD5 headers, 347, 514
Content-Range headers, 515
Content-Type headers, 348–351, 515
character encodings, 349
charset parameter, 371
META tags and, 375
MIME charset encoding tags and, 374
multipart form submissions, 349
continuation lines, in headers, 51
Continue status code (100), 59–60
Cookie headers, 516
Cookie2 headers, 516
cookies, 263–276
browsers, storage on, 264
caching, 273
domain attributes, 267
functioning, 264
information contained in, 264
Path attributes, 268
privacy and, 275
security and, 275
session tracking, 272
Set-Cookie2 headers, 271
specifications, 268
third-party vendors, use by, 267
types, 264
Version 0, 269
Version 1, 270–272
headers, 272
version negotiation, 272
web site specificity of, 266
COPY method, 442
country codes, 388
country tokens, 388
crawlers, 215–224
aliasing, 219
canonicalizing of URLs, 220
checkpoints, 219
cycles, avoiding, 217–218, 222–224
dups, 218
filesystem link cycles, 220
hash tables, 218
loops, 217
lossy presence bit maps, 218
partitioning, 219
root set, 216
search trees, 218
tracking of visited sites, 218
traps, 220–224
CRLF, 44
in entities, 343
cryptographic checksums, 289
cryptography, 309–317
asymmetrically keyed ciphers, 315
cipher machines, 311
ciphers, 310–315
digital, 311
ciphertext, 310
cleartext, 310
enumeration attacks, 313
hybrid cryptosystems, 317
keyed ciphers, 311
keys, 311, 312
key length, 313
sharing, logistical aspects of, 315
public-key cryptography, 315–317
computation speed, 317
digital signing with, 318
RSA algorithm, 317
symmetric-key ciphers, 313
CURLs (configuration URLs), 465
cycles, avoiding (web robots), 217–218,
222–224
filesystem link cycles, 220
D
data formats, conversion, 135
date formats, 392
Date headers, 516
DAV headers, 431
compliance classes, 445
decomposing of URLs, 33
Index | 621
dedicated web hosting, 412
delayed acknowledgements, 83
DELETE method, 58, 441
delta encodings, 359, 365–367
server disk space and, 368
delta generators and appliers, 368
<depth> element, 434
Depth headers, 431
Destination headers, 431
dictionary attacks, 304
digcalc.c file, 578
digcalc.h file, 577
digest authentication, 286–306, 574–580
algorithms, 291–295
input data, 291
authentication process, 287
Authentication-Info directives, 576
Authorization directives, 575
basic authentication, compared to, 286
caching and, 302
digest calculations, 291
error handling, 301
H(A1) and H(A2) reference code, 577
handshakes, 290
headers, 300
MD5 and MD5-sess, 291
message-related data (A2), 293, 298
nonces, 289
password files, vulnerabilities of, 305
preemptive authorization, 296
protection space, 302
request and response digest reference
code, 577
revalidating a session, 295
rewriting URIs, 302
security, 286
security-related data (A1), 293
session, 295
symmetric authentication, 298
WWW-Authenticate directives, 574–575
(see also authentication)
digests, 288
algorithm input data, 291
collisions, 288
digital certificates, 319–322
public-key cryptography and, 320
server authentication, use for, 321
universal standard, lack of, 320
virtual hosting and, 328
X.509v3 certificates, 320
digital cryptography (see cryptography)
digital signatures, 317–319
example, 318
digtest.c file, 580
directory listings, 122
disabling, 123
discrete MIME types, 534
distance delays, 163
distributed objects (HTTP:NG), 249
DNS
caching, 456
DNS A record lookup, 467
redirection, 453–457
enhanced algorithms for, 457
multiple addresses and round-robin
address rotation, 455
resolvers, 453
round robin, 454, 455
load balancing with, 456
docroots (document roots), 120
private, 122
user home directory, 122
virtually hosted, 121
document access control, 132
document expiration, 175
setting, 182
document hit rate, 167
document roots (see docroots)
documents
age and freshness lifetime, 188
age-calculation algorithms, 189–194
caching, preventing, 182
freshness and aging algorithms, 187–194
heuristic expiration, 184
Domain Name Service (see DNS)
domain names, internationalization of, 392
downstream message flow, 44
dups (web robots), 218
dynamic content resource mapping, 123
E
egress proxies, 137
embedded web servers, 111
encodings, 372
chunked encodings, 345
content encodings, 345, 351–354
delta encodings, 359, 365–367
impact on server disk space, 368
fixed-width, 381
transfer encodings, 354–359
end-of-line sequence, 44
622 | Index
entities, 342
body length, determining, 346
Content-Length headers, 344–347
CRLF line, 343
entity bodies, 44, 47, 52, 343
MIME types, 348
entity digests, 347
entity headers (see under headers)
entity tags (see ETags)
enumeration attacks, 313
equals sign (=), base-64 encoding, 572
escape sequences, 36
ETags (entity tags), 180, 298
headers, 517
using, 181
euc-jp encoding, 383
Expect headers, 517
experimental MIME types, 569
expiration of documents, setting, 182
Expires headers, 175, 183, 517
explicit MIME typing, 126
Extensible Markup Language (see XML)
extension APIs, 205
extension headers, 51, 68
extension methods, 58
F
Fast CGI, 205
fat URLs, 262
limitations of, 263
file scheme, 39
filesystem link cycles, 220
FindProxyForURL( ) method, 143
fingerprint functions, 289
first subtag, 387
fixed-width encodings, 381
flash crowds, 163
format conversion, 404
FPAdminScriptUrl, 426
FPAuthorScriptUrl, 426
FPShtmlScriptUrl, 426
Fpsrvadm, 428
frag or fragment component, URLs, 30
freshness and aging algorithms, 187–194
freshness lifetime, 188
From headers, 258, 517
robots and, 225
FrontPage, 424–429
client and server extension
communication, 426
FrontPage Server Extensions (FPSE), 424
HTTP POST requests and, 425
listExploreDocs element, POST request
body, 427
listHiddenDocs element, POST request
body, 427
root web, 425
RPC protocol, 426
security, 428
server administrator utility
(Fpsrvadm), 428
service_name element, POST request
body, 427
subweb, 426
virtual servers, 425
ftp scheme, 39
full NAT, 461
full-text indexes, 243
G
gateways, 19, 197–205
client- and server-side, 199
client-side security accelerator
gateways, 202
examples, 198
protocol gateways, 200
proxies, contrasted with, 130
resource gateways, 203
server-side security gateways, 202
server-side web gateways, 200
Via headers and, 153
general headers (see under headers)
general-purpose software web servers, 110
Generic Router Encapsulation (GRE), 472
GET command, virtual hosting issues, 414
GET messages, processing steps, 171–175
GET method, 53
getpeername function, 259
glyphs, 378
GRE (Generic Router Encapsulation), 472
H
H(A1) and H(A2) reference code, 577
half NAT, 461
handshake delays, 82
handshakes, digest authentication, 290
hash tables, 218
H(d), one-way hash, 291
HEAD method, 54
HEAD response, 346
Index | 623
headers, 47, 51, 67–73, 508–532
Accept, 69, 508
robots and, 225
Accept-Charset, 371, 375, 509
MIME charset encoding tags and, 374
Accept-Encoding, 509
Accept-Language, 371, 385, 510
content negotiation and, 398
Accept-Ranges, 510
Age, 510
Allow, 159, 511
authentication, 278
basic, 281, 300
digest, 300
Authentication-Info directives, 576
Authorization, 281, 511
directives, 575
preemptive generation, 295
Cache-Control, 175–177, 182–186, 189,
511
directives, 361
character set requirements, 392
classification, 51
client identification using, 258
Client-ip, 258, 260, 512
Connection, 86, 512
content negotiation, 397
Content-Base, 513
Content-Encoding, 513
Content-Language, 371, 384, 513
Content-Length, 344–347, 514
Content-Location, 514
Content-MD5, 347, 514
Content-Range, 515
Content-Type, 348–351, 515
charset parameter, 371
continuation lines, 51
Cookie, 516
Cookie2, 516
Date, 516
DAV, 431
compliance classes, 445
Depth, 431
Destination, 431
entity headers, 51, 67, 72
content headers, 72
entity caching headers, 73
HTTP/1.1, 342
ETag, 517
examples, 51
Expect, 517
Expires, 175, 183, 517
extension headers, 68
From, 517
robots and, 225
general headers, 51, 67, 68
caching headers, 68
Heuristic Expiration Warning, 184
Host, 417, 418, 419, 518
robots and, 225
HTCP cache headers, 480
If, 431
If-Match, 519
If-Modified-Since, 166, 178, 518
If-None-Match, 180, 519
If-Range, 519
If-Unmodified-Since, 520
Last-Modified, 520
Location, 520
Lock-Token, 431
max-age, 183
Max-Forwards, 155, 521
for media types, 348
Meter, 196, 493
MIME-Version, 521
must-revalidate, 183
no-cache, 182
no-store, 182
Overwrite, 432, 442
Pragma, 68, 182, 521
Proxy-Authenticate, 522
Proxy-Authorization, 522
Proxy-Connection, 96, 523
Public, 523
Range, 524
Referer, 259, 524
robots and, 225
request headers, 51, 67, 69–71
accept headers, 69
client identification using, 258
conditional request headers, 70
proxy request headers, 70
request security headers, 70
response headers, 51, 67, 71–72
negotiation headers, 71
response security headers, 72
Retry-After, 525
Server, 525
Set-Cookie, 264, 525
caching and, 273
domain attributes, 267
Set-Cookie2, 271, 526
624 | Index
headers (continued)
syntax, 51
tampering attack, 303
TE, 526
Timeout, 432, 435
Title, 527
Trailer, 526
Transfer-Encoding, 527
UA-, 527
for uncachable documents, 182
unsupported headers, handling, 158
Upgrade, 528
User-Agent, 225, 259, 528
Vary, 402, 529
Via, 151–154, 529
Want-Digest, 348
Warning, 530
Warning 13, 184
for WebDAV, 431
WWW-Authenticate, 281, 531, 574–575
X-Cache, 531
X-Forwarded-For, 260, 531
X-Pad, 531
X-Serial-Number, 532
heartbeat messages, 473
heuristic expiration of documents, 184
Heuristic Expiration Warning headers, 184
history expansion, 34
hit logs, 195
hit metering, 492–494
Meter headers, 493
hit rate, 167
host component, URLs, 27
Host headers, 417, 418, 518
clients, requirements for, 418
missing host headers, 419
proxies and, 418, 419
robots and, 225
web servers, interpretation by, 419
hostile proxies, 304
hosting services, 411
dedicated web hosting, 412
hostname expansion, 34
hostnames, 13
<href> element, 438
.htaccess, 428
HTCP (Hyper Text Caching
Protocol), 478–481
authentication, 480
cache headers, 480
caching policies, setting, 480
data components, 479
message structure, 478
opcodes, 480
HTML (Hypertext Markup Language)
displaying resources using HTTP, 13
documents, relative URLs in, 31
fragments, referencing, 30
robot-control META tags, 237
HTTP (Hypertext Transfer Protocol), xiii,
3–11, 247
authentication, challenge/response
framework, 278
(see also authentication)
authentication schemes, security
risks, 303
base-64 encoding, compatibility
with, 570
caching (see caches)
character sets (see character sets)
clients and servers, 4
commands, 8
CONNECT method, 206–208
connections (see HTTP connections)
entities (see entities)
headers (see headers)
hit metering extension, 492
HTTP-NG (see HTTP-NG)
informational resources, 21
instance manipulations, 359
international content support, 370
limitations, 248
messages (see HTTP messages)
methods, 8
performance considerations (see under
TCP)
proxy servers (see HTTP proxy servers)
redirection, 452–453
relays, 212
reliability of, 3
revalidations, 165–166
robots, standards for, 225
secure HTTP (see HTTPS)
status codes, 9, 505–507
TCP, dependency on, 80
textual basis of, 10
transactions, 8
delays, causes of, 80
truncation detection, 344
versions, 16
HTTP connections, 74, 75, 86–104
closing, 101–104
Connection headers, 86, 512
establishing, 13, 15–16
Index | 625
keep-alive connections
(HTTP/1.0+), 91–96
parallel (see parallel connections)
persistent (see persistent connections)
pipelined connections, 99
Proxy-Connection headers, 96, 523
serial loading, 87
(see also TCP)
HTTP messages, 8, 10, 43–73
entity bodies, 47, 52
example, 11
flow, 43
GET messages, processing steps, 171–175
headers (see headers)
methods (see methods)
reason phrases, 47, 50
redirection of, 450
request lines, 48
robots, setting conditions for, 226
request URLs, 46
response lines, 48
robots, handling by, 227
start lines, 47–51
status codes (see status codes)
structure, 44
syntax, 45
tracing across proxies, 150–157
Via headers, 151–154
Version 0.9 messages, 52
versions, 46, 50
HTTP proxy servers, 129–160
authentication, 156
client proxy configuration, 141–144
client traffic acquisition, 140
deploying, 137
interoperation, 157–160
messages, tracing, 150–157
proxy and server requests, handling, 146
proxy hierarchies, 138
public and private proxies, 130
TRACE method and network
diagnosis, 155
unsupported headers and methods,
handling, 158
URIs, in-flight modification of, 147
uses, 131–136
http scheme, 38
HTTP State Management Mechanism, 265
HTTP: The Next Generation (see HTTP-NG)
HTTP/0.9, 16
HTTP/1.0, 16
server requests to virtual hosts, problems
with, 413
HTTP/1.0+, 17
HTTP/1.1, 17
enhanced methods, 444
entity header fields, 342
Host headers (see Host headers)
limitations, 248
request pipelining, 99
TRACE method, 155
HTTP/2.0, 17
HTTP-EQUIV tag, 187
HTTP-NG (HTTP: The Next
Generation), 17, 248–253
current status, 252
message transport layer, 249, 250
modularization, 248
object types, 252
remote invocation layer, 249, 250
web application layer, 249, 251
HTTPS, 76, 308–309, 322–336
authentication, 326
clients, 328–335
OpenSSL example, 329–335
CONNECT method, HTTP, 206–208,
336
connecting, 324
default port, 323
OpenSSL, 328–335
schemes, 308, 323
site certificate validation, 327
SSL handshake, 324–326
tunnels (see tunnels)
https scheme, 38
Hyper Text Caching Protocol (see HTCP)
Hypertext Markup Language (see HTML)
Hypertext Transfer Protocol (see HTTP)
I
IANA (Internet Assigned Numbers
Authority)
instance manipulations, registered
types, 367
MIME charset registry, 602–615
MIME type registration, 537–539
registered language tags, 386, 582
ICP (Internet Cache Protocol), 473
vs. CARP, 475
ident protocol, 115
626 | Index
If headers, 431
If-Match headers, 519
If-Modified-Since headers, 166, 178, 518
If-None-Match headers, 180, 519
If-Range headers, 519
If-Unmodified-Since headers, 520
image/* MIME types, 561–562
inbound messages, 43
inbound proxies, 138
indexes, full-text, 243
informational status codes (100-199), 59–60,
505
ingress proxies, 137
instance manipulations, 359, 367–369
delta encodings, 365
IANA registered types, 367
range requests, 364
integrity protection, 299
intercepting proxies, 140, 146
URI resolution with, 149
internationalization
date formats, 392
domain names, 392
headers, character set for, 392
ISO 3166 country codes, 594–600
ISO 639 language codes, 583–594
languages, administrative
organizations, 601
URL variants, 395
Internet Assigned Numbers Authority (see
IANA)
Internet Cache Protocol (ICP), 473
Internet search engines (see search engines)
IP address forwarding, 460
IP (Internet protocol) addresses, 13
clients, identification using, 259
virtual hosting and, 416
IP MAC forwarding, 459
IP packets, 76
iPlanet web servers, 110
ISO 3166 country codes, 594–600
ISO 639 language codes, 583–594
iso-2022-jp encoding, 382
iso-8859 character set, 380
J
Japanese encodings, 382, 383
JIS X 0201, 0208, and 0212 character
sets, 380
Joe’s Hardware Store web site, xiv
K
KD(s,d) digest, 291
keep-alive connections (HTTP/1.0+), 91–96
(see also persistent connections)
keyed ciphers, 311
keys, 311, 312
key length, 313
L
language preferences, configuring, 389
language tags, 370, 384, 581–600
case sensitivity, 386
first subtag rules, 581
IANA-registered tags, 386, 582
reference tables, 389
second subtag rules, 582
subtags, 386
syntax, 385
Last-Modified dates, using, 181
Last-Modified headers, 520
layering of protocols, 12
layout delay, preventing, 88
ligatures, 378
listExploreDocs element, POST request
body, 427
listHiddenDocs element, POST request
body, 427
LM-Factor algorithm, 184
load balancing, 449
DNS round robin, 454–457
single clients and, 456
loading, serial, 87
Location headers, 520
LOCK method, 433
status codes, 436
lock refreshes, 435
<lockdiscovery> element, 435
<lockinfo> element, 434
locking, 433
<locktoken>element, 434
Lock-Token headers, 431
logging, 483–492
commonly logged fields, 483
interpretation, 484
log formats, 484–492
Combined Log Format, 485
Common Log Format, 484
Netscape Extended 2 Log
Format, 487–489
Index | 627
Netscape Extended Log Format, 486
Squid Proxy Log Format, 489–492
privacy concerns, 495
loops (web robots), 217
M
MAC (Media Access Control) addresses, 459
magic typing, 126
mailto scheme, 38
man-in-the-middle attacks, 304
manual client proxy configuration, 142
master origin server, 420
max-age response headers, 183
Max-Forwards headers, 155, 521
MD5, 288, 291, 293, 347
MD5-sess, 291, 293
Media Access Control (MAC) addresses, 459
media types, 348
multipart, 349
message body, 44
message digest algorithms, 291–294
symmetric authentication, 298
message integrity protection, 299
message/* MIME types, 563
message transport layer (HTTP-NG), 249,
250
message truncation, 344
messages (see HTTP messages)
<META HTTP-EQUIV> tag, 187
META tag directives, 239
meta-information, 43
Meter headers, 196, 493
methods, 46, 48, 53–59
CONNECT, 206–208, 336
DELETE, 58, 441
extension methods, 58
GET, 53
HEAD, 54
OPTIONS, 57, 159, 445
POST, 55
PUT, 54, 444
redirection status codes and, 64
TRACE, 55
unsupported, handling, 158
Microsoft FrontPage (see FrontPage)
Microsoft Internet Explorer
cookie storage, 266
language preference configuration, 389
Microsoft web servers, 110
MIME (Multipurpose Internet Mail
Extensions)
charset encoding tags, 374
charset registry, 602–615
preferred MIME names, 603
“multipart” email messages, 349
(see also MIME types)
MIME types, 5, 533–569
application/* types, 540–557
audio/* types, 557–559
chemical/* types, 559–561
composite types, 534
discrete types, 534
documentation, 534
experimental types, 569
IANA registration, 537–539
media type registry, 539
process, 537
registration trees, 537
rules, 538
template, 538
image/* types, 561–562
message/* types, 563
model/* types, 563
multipart/* types, 535, 564
primary types, 536
structure, 534
syntax, 536
tables, 539–569
text/* types, 565–568
video/* types, 568
MIME typing, 125
MIME::Base64 Perl module, 572
MIME-Version headers, 521
mirrored server farms, 420
MKCOL method, 440
mod_cern_meta module, Apache web
server, 186
model/* MIME types, 563
mod_expires module, Apache web
server, 186
mod_headers module, Apache web
server, 186
MOVE method, 442
multi-homed servers, 425
multipart form-data encodings, 349
multipart/* MIME types, 535, 564
multiplexed architectures, 119
I/O web servers, 119
multithreaded web servers, 119
628 | Index
multiprocess, multithreaded web
servers, 118
Multipurpose Internet Mail Extensions (see
MIME; MIME types)
<multistatus> element, 438
MultiViews directive, 400
must-revalidate response headers, 183
N
Nagle’s algorithm, 84
namespace management, 439–444
methods used for, 440
status codes, 443
namespaces, 388
language subtags, 387–389
XML, 430
NAT (Network Address Translation), 460
NECP (Network Element Control
Protocol), 461
negotiation headers, 71
Netscape Extended 2 Log Format, 487–489
Netscape Extended Log Format, 486
Netscape Navigator
cookies
storage, 265
Version 0, 269
language preference configuration, 389
Network Address Translation (NAT), 460
network bottlenecks, 161
Network Element Control Protocol
(NECP), 461
network exchange proxies, 137
news scheme, 39
no-cache response headers, 182
nonces, 289–298
next nonce pregeneration, 297
reuse, 297
selection, 298
time-synchronized generation, 297
no-store response headers, 182
.nsconfig, 428
O
object types, HTTP-NG, 252
one-way digests, 288
one-way hashes, 291
functions, 289
opaquelocktoken scheme, 433, 434
OpenSSL, 328–335
example client, 329–335
OPTIONS method, 57, 445
requests, 159
response headers to, 445
origin servers, 420
outbound messages, 43
outbound proxies, 138
Overwrite headers, 432, 442
P
PAC files, 142
autodiscovery, 465
PAC (Proxy Auto-Configuration)
protocol, 463
parallel connections, 88–90
impression of speed, 90
loading speed, 88
open connection limits, 90
persistent connections vs., 91
parameters component, URLs, 28
parent and child relationships, 138
parent caches, 169
password component, URLs, 27
passwords
digest authentication password file,
risks, 305
digest authentication, security, 287
path component, URLs, 28
Perl code for interaction with robots.txt
files, 235
Perl web server, 111
persistent connections, 90–99
Content-Length headers and, 345
keep-alive connections
(HTTP/1.0+), 91–96
parallel connections vs., 91
restrictions and rules, 98
persistent uniform resource locators
(PURLs), 40
pipelined connections, 99
plaintext, security and, 310
port component, URLS, 27
port exhaustion, 85
port numbers, 13, 77
default values, 13
virtual hosting and, 415
POST method, 55
POST requests, FrontPage and, 425
Pragma headers, 68, 182, 521
Pragma: no-cache headers, 182
precompiled dictionary attacks, 305
Index | 629
preemptive authorization, 295
presence bit arrays (web robots), 218
presentation forms, 378
primary subtags, 386
privacy, 495
cookies and, 275
robots and, 229
private caches, 168
private docroots, 122
private proxies, 130
<prop> element, 437
<propertyupdate> element, 438
PROPFIND method, 437
server response elements, 438
XML elements, used with, 437
<propname> element, 437
PROPPATCH method, 438–439
XML elements, used with, 438
<propstat> element, 438
“protecting the header”, 87
protection spaces, 295, 301
protocol gateways, 200
protocol stack, 76
protocols, layering of, 12
proxies
100 Continue status code and, 60
authentication, 156
deploying, 137
egress proxies, 137
gateways, contrasted with, 130
hostile, 304
HTTP proxies (see HTTP proxy servers)
ingress proxies, 137
intercepting proxies, 140, 146
URI resolution with, 149
interoperation, 157–160
messages, tracing, 150–157
“missing scheme/host/port”
problem, 146
proxy and server requests, handling, 146
proxy hierarchies, 138
content routing, 139
dynamic parent selection, 140
public and private, 130
redirection and, 449
surrogates, 146
transparent negotiation, 400
transparent proxies, 140
tunnels and, 335
URIs, in-flight modification of, 147
uses, 131–136
(see also HTTP proxy servers)
Proxy-Authenticate headers, 522
proxy authentication, 283
Proxy-Authorization headers, 522
Proxy Auto-configuration (PAC)
protocol, 142, 463
(see also PAC files)
proxy cache hierarchies, 169
proxy caches, 169
Proxy-Connection headers, 96, 523
proxy redirection
CARP (Cache Array Routing
Protocol), 475–478
HTCP (Hyper Text Caching
Protocol), 478–481
ICP (Internet Cache Protocol), 473
proxy redirection methods, 462–469
explicit browser configuration, 463
disadvantages, 463
PAC (Proxy Auto-configuration)
protocol, 463
WPAD (see WPAD)
proxy request headers, 70
proxy servers, 18
networks, use in securing, 335
(see also HTTP proxy servers)
proxy URIs vs. server URIs, 144
public caches, 169
Public headers, 523
public proxies, 130
public-key cryptography (see under
cryptography)
publishing systems
FrontPage (see FrontPage)
WebDAV (see WebDAV)
(see also web publishing)
PURLs (persistent uniform resource
locators), 40
PUT method, 54, 444
Q
qop (quality of protection), 293
enhancements, 299
quality factors, 371
quality of protection (see qop)
quality values, 398
query component, URLs, 29
630 | Index
R
Range headers, 524
range requests, 363
realm directive, 280
realms (protection spaces), 301
reason phrases, 9, 47, 50, 505–507
redirection, 126, 448–481
anycast addressing, 457
enhanced DNS-based algorithms, 457
IP address forwarding, 460
IP MAC forwarding, 459
load balancing and, 127, 449
methods, 450
DNS redirection, 453–457
HTTP redirection, 452–453
proxy methods, 462–469
proxy techniques, 451
NECP (Network Element Control
Protocol), 461
protocols, 450
proxies, role in, 449
purpose, 449
techniques, 448
temporary redirect, 126
transparent redirection, 469
URL augmentation, 126
WCCP (Web Cache Coordination
Protocol), 470–473
redirection status codes (300-399), 61–64,
505
Referer headers, 259, 524
robots and, 225
relative URLs, 30–34
bases, 31
resolving, 33
relays, 212
keep-alive connections and, 212
relevancy ranking, 245
reliable bit pipe, 75
remote invocation layer (HTTP-NG), 249,
250
remote procedure call (RPC) protocol,
FrontPage, 426
<remove> element, 439
replay attacks, 284, 289, 303
preventing, 289
replica origin servers, 420
request digest reference code, 577
request digests, 294
request headers (see under headers)
request lines, 48
request messages, 10, 45, 47
methods, 46
request URLs, 46
request method (HTTP), 294
request pipelining, 99
request security headers, 70
reserved characters (see restricted characters)
resource gateways, 203
resource locator servers, 40
resource paths, 24
resources, mapping and accessing of, 120
response digest reference code, 577
response entities, 125
response headers (see under headers)
response lines, 48
response messages, 10, 45, 125
restricted characters, 36
Retry-After headers, 525
revalidate hits, 165
revalidate misses, 166
revalidations, 165–166
reverse proxies, 134, 137
RobotRules object, 235
robots
conditional HTTP requests, 226
entities and, 227
etiquette, 239–241
excluding from web sites, 229–239
HTML robot control META tags, 237
HTTP and, 225
request headers, identifying, 225
META directives, 237
Meta HTML tags and, 227
privacy and, 229
problems caused by, 228
response handling, 227
search engines, 242–246
status codes, handling of, 227
(see also robots.txt files)
Robots Exclusion Standard, 230
robots.txt files, 229
caching and expiration, 234
comments, 234
disallow and allow lines, 233
example, 236
fetching, 231
records, 232
specification, changes in, 234
status codes for retrievals, 231
syntax, 232
User-Agent line, 232
web sites and, 231
Index | 631
root set, 216
root web, 425
round-robin load balancing, 453
DNS round robin, 454–457
routers and anycast addressing, 457
RPC protocol, FrontPage, 426
RSA algorithm, 317
rtsp, rtspu schemes, 39
S
schemes, 7, 24, 27
common formats, 38
URIs for, 499–504
search engines, 242–246
architecture, 242
full-text indexes, 243
queries, 244
relevancy ranking, 245
results, sorting and formatting, 244
spoofing, 245
search trees, 218
second subtag, 388
Secure Sockets Layer (see SSL)
security
basic authentication and, 283
cookies and, 275
digest authentication and, 286
firewalls, 132
FrontPage, security model, 428
HTTP authentication schemes, associated
risks, 303
key length and, 314
multiple authentication schemes,
risks, 303
security realms, 280
WPAD security hole, 468
segments, 76
“sender silly window syndrome”, 84
serial loading, 87
serial transaction delays, 87
server error status codes (500-599), 66, 505
server farms, 413
mirrored servers, 420
Server headers, 525
Server response header field, 154
server URIs vs. proxy URIs, 144
server-driven negotiation, 397–400
server-side extensions, 400
servers
100 Continue status codes and, 60
accelerators, 134
certificates, 326
delta encodings, impact on, 368
error status codes (500-599), 66
extension APIs, 205
FrontPage Server Extensions (FPSE), 424
Host headers, interpreting, 419
multi-homed servers, 425
revalidation, 177
server farms, 413
master origin servers, 420
replica origin servers, 420
supported functionality, identifying, 159
validation, 175
server-side extensions, 400
server-side gateways, 199
security gateways, 202
web gateways, 200
server-side includes (SSIs), 124
service groups, 472
service_name element, POST request
body, 427
sessions, cookies and, 264
tracking with, 272
<set> element, 438
Set-Cookie headers, 264, 525
caching and, 273
domain attributes, 267
Set-Cookie2 headers, 271, 526
shared hosting, 413–419
shared keys, 315
shared proxies, 130
sibling caches, 171
Simple Object Access Protocol (SOAP), 206
single-threaded web servers, 118
site certificate validation, 327
slow hits, 165
s-maxage response headers, 183
SOAP (Simple Object Access Protocol), 206
sockets API, 78
calls, 78
software web servers, 110
spiders, 215
spoofing, 245
Squid Proxy Log Format, 489–492
SSIs (server-side includes), 124
SSL (Secure Sockets Layer), 308
authentication, 326
handshakes, 324–326
HTTPS, integration in, 323
OpenSSL, 328–335
site certificate validation, 327
tunnels, 209
vs. HTTP/HTTPS gateways, 210
632 | Index
SSLeay, 329
(see also OpenSSL)
start lines, 47–51
status codes, 9, 49, 59–67
classes, 49
client error codes (400-499), 65–66, 505
HTTP codes, 505–507
informational status codes
(100-199), 59–60, 505
LOCK method, 436
namespace management methods, 443
redirection status codes
(300-399), 61–64, 505
robots, handling by, 227
server error codes (500-599), 66, 505
success status codes (200-299), 61, 505
UNLOCK method, 436
<status> element, 438
strong validators, 181, 363
subtags, 386, 389
first subtag, 387
second subtag, 388
subweb, 426
success status codes (200-299), 61, 505
surrogate caches, 421
surrogate proxies, 137
surrogates, 134, 146
symbolic links and cycles, 220
symmetric authentication, 298
symmetric-key ciphers, 313
syntax, headers, 51
T
TCP slow start (see under TCP)
TCP (Transmission Control
Protocol), 74–86
connections, 75
distinguishing values, 77
establishing, 13, 79
web server handling of, 115
(see also HTTP connections)
network delays, causes, 80
performance considerations, 80–86
connection handshake delays, 82
delayed acknowledgements, 83
delays, most common causes, 81–86
Nagle’s algorithm, 84
port exhaustion, 85
TCP slow start, 83
TCP_NODELAY, 84
TIME_WAIT accumulation, 85
port numbers and, 77
reliability, 74
serial loading, 87
sockets API, 78
TCP slow start, 83
TCP/IP (Transmission Control
Protocol/Internet Protocol), 11
TCP_NODELAY parameter, 84
TE headers, 526
Telnet example, 15–16
telnet scheme, 40
text/* MIME types, 565–568
<timeout> element, 434
Timeout headers, 432, 435
TIME_WAIT accumulation, 85
Title headers, 527
TLS (Transport Layer Security), 308
(see also SSL)
TRACE method, 55, 155–157
Max-Forwards headers and, 155
Trailer headers, 526
transactional direction, messages, 43
transactions, 8
transcoders, 135
transcodings, 395, 403–406
content injection, 405
format conversion, 404
information synthesis, 404
types, 404
vs. static pregenerated content, 405
transfer encodings, 354–359
Transfer-Encoding headers, 527
Transmission Control Protocol/Internet
Protocol (TCP/IP), 11
transparent negotiation, 400–403
caching, 401
Vary headers, 402
transparent proxies, 140
transparent redirection, 469
Transport Layer Security (see TLS)
trees, 218
truncation detection, 344
tunnels, 19, 206–212
authentication, 211
HTTPS SSL, 335–336
security, 211
SSL tunnels vs. HTTP/HTTPS
gateways, 210
type negotiation, 126
type-map files, 399
type-o-serve web server, 112
Index | 633
U
UA- headers, 527
UCS (Universal Character Set), 381
uncachable documents, 182
uniform resource identifiers (see URIs)
uniform resource locators (see URLs)
uniform resource names (see URNs)
Universal Character Set (UCS), 381
UNLOCK method, 435
status codes, 436
Upgrade headers, 528
upstream message flow, 44
uri-directive-value, 294
URIs (uniform resource identifiers), 6, 24
client autoexpansion and hostname
resolution, 147
intercepting proxies, resolution with, 149
internationalization, 389–391
resolution, 144–150
proxy vs. server, 144
with a proxy, explicit, 149
without a proxy, 148
rewriting, 302
schemes, 499–504
URLs (uniform resource locators), 6, 23–42
advantages of, 25
aliases, 219
augmentation, 126
automatic expansion, 34
canonical form, 37
canonicalizing, 220
character sets, 35, 35–38
CURLs, 465
examples, 13
fat URLs, client identification using, 262
informational resources, 41
portability, 35
PURLs, 40
relative URLs, 30–34
restricted characters, 36
schemes, 7
schemes (see schemes)
shortcuts, 30
structure, 24
syntax, 26–30
URIs, as a subset of, 24
variants, 395
virtual hosting, paths, 415
URNs (uniform resource names), 7, 40
standardization, 41
US-ASCII character set, 379
user agents, 19
user component, URLs, 27
user home directory docroots, 122
User-Agent headers, 225, 259, 528
user-tracking systems, content injection
and, 405
UTF-8 encoding, 382
V
validators, 362
Last-Modified dates, using, 181
strong and weak, 181, 363
variable-length codes, 372
variable-width modal encodings, 381
variable-width nonmodal encodings, 381
Vary headers, 402, 529
Vermeer Technologies, Inc., 424
version numbers, HTTP messages, 50
Via headers, 151–154, 529
gateways and, 153
privacy and security, 154
request and response paths, 153
Server response header fields and, 154
syntax, 152
video/* MIME types, 568
virtual hosting, 225, 413–419
digital certificates and, 328
docroots, 121
GET command, problems with, 414
Host headers, by, 417
(see also Host headers)
IP addresses, by, 416
problems for hosters, 417
port numbers, by, 415
server requests, absence of host
information in HTTP/1.0, 413
fixes, 415
URL paths, by, 415
virtual servers, 425
W
Want-Digest headers, 348
Warning headers, 530
Heuristic Expiration Warning, 184
WCCP (Web Cache Coordination
Protocol), 470–473
GRE packet encapsulation, 472
heartbeat messages, 473
load balancing, 473
operation, 470
634 | Index
WCCP (continued)
service groups, 472
versions, 470
WCCP2 messages, 470–472
header and components, 471
weak validators, 181, 363
web application layer (HTTP-NG), 249, 251
web architecture, 17–20
Web Cache Coordination Protocol (see
WCCP)
web caches, 18, 161
(see also caches)
web clients and servers, 4
Web Distributed Authoring and Versioning
(see WebDAV)
web hosting, 411–422
hosting services, 411
shared or virtual hosting, 413–419
web pages, 9
Web Proxy Autodiscovery Protocol (see
WPAD)
web proxy servers, 129
web publishing
collaborative authoring, 429
publishing systems, 424
web resources, 4
web robots, 215–246
crawlers (see crawlers)
examples, 215
spiders, 215
web servers, 109
access controls, 124
appliances, 111
client hostname identification, 115
client identification, 257–276
cookies, using, 263–276
fat URLs, using, 262
headers, using, 258
IP address, using, 259
user login, using, 260
connection input/output processing
architectures, 117
connections, handling new, 115
directory listings, 122
docroots, 120
dynamic content resource mapping, 123
embedded web servers, 111
explicit typing, 126
HTTP proxy servers (see HTTP proxy
servers)
ident protocol, 115
implementations, 109
logging, 127
MIME typing, 125
multiplexed I/O servers, 119
multiplexed multithreaded servers, 119
multiprocess, multithreaded servers, 118
Perl example, 111
redirection responses, 126
request handling, 449
request messages
receiving, 116
structure of, 117
resources, mapping and accessing of, 120
response entities, 125
response messages, 125
responses, sending, 127
single-threaded servers, 118
software web servers, 110
SSIs (server-side includes), 124
tasks of, 113–114
type negotiation, 126
type-o-serve, 112
user authentication (see authentication)
web services, 206
web sites
personalizing of user experience, 257
reliability, improving, 419–422
mirrored server farms, 420
robots, exclusion, 229–239
robots.txt files, 231
speed, improving, 422
web tunnels (see tunnels)
WebDAV (Web Distributed Authoring and
Versioning), 429–446
collaborative authoring and, 429
collections, 439–444
DAV header, 431
Depth header, 431
Destination header, 431
enhanced HTTP/1.1 methods, 444
headers, 431
If header, 431
LOCK method, 433
locking, 432
Lock-Token headers, 431
META data, embedding, 436–439
methods, 429
namespace management, 439–444
OPTIONS method, 445
Overwrite headers, 432
overwrites, preventing, 432
Index | 635
PROPATCH method, 438–439
properties, 436–439
PROPFIND method, 437
PUT method, 444
Timeout headers, 432
UNLOCK method, 435
version management, 446
XML and, 430
WebMUX protocol, 250, 251
WPAD (Web Proxy Autodiscovery
Protocol), 143, 464–469
administration, 469
CURLs, 465
DHCP discovery, 467
DNS A record lookup, 467
PAC file retrieval, 467
PAC file autodiscovery, 465
resource-discovery algorithm, 143, 465
spoofing, 468
timeouts, 468
timing, 468
WWW-Authenticate headers, 281, 531
directives, 574–575
WWW::RobotRules object, 235
X
X.509v3 certificates, 320
X-Cache headers, 531
X-Forwarded-For headers, 260, 531
XML (Extensible Markup Language), 206,
430
elements used in locking, 434
namespace, 430
schema definition, XML documents, 430
WebDAV and, 430
X-Pad headers, 531
X-Serial-Number headers, 532
About the Authors
David Gourley is the Chief Technology Officer of Endeca, where he leads the
research and development of Endeca’s products. Endeca develops Internet and
intranet information-access solutions that provide new ways to navigate and explore
enterprise data. Prior to working at Endeca, David was a member of the founding
engineering team at Inktomi, where he helped develop Inktomi’s Internet search
database and was a key developer of Inktomi’s web caching products.
David earned a B.A. in Computer Science from the University of California at
Berkeley, and he holds several patents in web technologies.
Brian Totty was most recently the Vice President of R&D at Inktomi Corporation (a
company he helped found in 1996), where he led research and development of web
caching, streaming media, and Internet search technologies. Formerly, he was a
scientist at Silicon Graphics, where he designed and optimized software for high-
performance networking and supercomputing systems. Before that, he held an engi-
neering position at Apple Computer’s Advanced Technology Group.
Brian holds a Ph.D. in Computer Science from the University of Illinois at Urbana-
Champaign and a B.S. degree in Computer Science and Electrical Engineering from
MIT, where he received the Organick award for computer systems research. He also
has developed and taught award-winning courses on Internet technology for the
University of California Extension system.
Marjorie Sayer writes about network caching software at Inktomi Corporation. After
earning M.A. and Ph.C. degrees in Mathematics at the University of California at
Berkeley, she worked on mathematics curriculum reform. Since 1990 she has written
about energy resource management, parallel systems software, telephony, and
networking.
Sailu Reddy currently leads the development of embedded performance-enhancing
HTTP proxies at Inktomi Corporation. Sailu has been developing complex software
systems for 12 years and has been deeply involved in web infrastructure research and
development since 1995. He was a core engineer of Netscape’s first web server and
web proxy products and of several following generations. His technical experience
includes HTTP applications, data compression techniques, database engines, and
collaboration management. Sailu earned an M.S. in Information Systems from the
University of Arizona and holds several patents in web technologies.
Anshu Aggarwal is a Director of Engineering at Inktomi Corporation. He leads the
protocol-processing engineering teams for Inktomi’s web caching products, and he
has been involved in the design of web technologies at Inktomi since 1997. Anshu
holds M.S. and Ph.D. degrees in Computer Science from the University of Colorado
at Boulder, specializing in memory-consistent techniques for distributed multipro-
cessor machines. He also holds M.S. and B.S. degrees in Electrical Engineering.
Anshu is the author of several technical papers and holds two patents.
Colophon
Our look is the result of reader comments, our own experimentation, and feedback
from distribution channels. Distinctive covers complement our distinctive approach
to technical topics, breathing personality and life into potentially dry subjects.
The animal on the cover of HTTP: The Definitive Guide is a thirteen-lined ground
squirrel (Spermophilus tridecemlineatus), common to central North America. True to
its name, the thirteen-lined ground squirrel has thirteen stripes with rows of light
spots that run the length of its back. Its color pattern blends into its surroundings,
protecting it from predators. Thirteen-lined ground squirrels are members of the
squirrel family, which includes chipmunks, ground squirrels, tree squirrels, prairie
dogs, and woodchucks. They are similar in size to the eastern chipmunk but smaller
than the common gray squirrel, averaging about 11 inches in length (including a 5–6
inch tail).
Thirteen-lined ground squirrels go into hibernation in October and emerge in late
March or early April. Each female usually produces one litter of 7–10 young each
May. The young leave the burrows at four to five weeks of age and are fully grown at
six weeks. Ground squirrels prefer open areas with short grass and well-drained
sandy or loamy soils for burrows, and they avoid wooded areas—mowed lawns, golf
courses, and parks are common habitats.
Ground squirrels can cause problems when they create burrows, dig up newly
planted seeds, and damage vegetable gardens. However, they are important prey to
several predators, including badgers, coyotes, hawks, weasels, and various snakes,
and they benefit humans directly by feeding on many harmful weeds, weed seeds,
and insects.
Rachel Wheeler was the production editor and copyeditor for HTTP: The Definitive
Guide. Leanne Soylemez, Sarah Sherman, and Mary Anne Weeks Mayo provided
quality control, and Derek Di Matteo and Brian Sawyer provided production assis-
tance. John Bickelhaupt wrote the index.
Ellie Volckhausen designed the cover of this book, based on a series design by Edie
Freedman. The cover image is an original illustration created by Lorrie LeJeune.
Emma Colby produced the cover layout with QuarkXPress 4.1 using Adobe’s ITC
Garamond font.
David Futato and Melanie Wang designed the interior layout, based on a series
design by David Futato. Joe Wizda prepared the files for production in FrameMaker
5.5.6. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed;
and the code font is LucasFont’s TheSans Mono Condensed. The illustrations that
appear in the book were produced by Robert Romano and Jessamyn Read using
Macromedia FreeHand 9 and Adobe Photoshop 6. This colophon was written by
Rachel Wheeler.

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